maanantai 20. helmikuuta 2012

Moving towards perfection

We saw last time the ideal of knowledge in Aristotle, but what is still lacking is the actual knowledge. Now, whereas in Plato there was one overreaching question – how to live a good life – governing whole of his philosophy, Aristotelian philosophy was governed by several questions. First of them might be: ”what is there and what is it like?” The aim of of this question was to find a vision of things and their essences: vision or theoria, which is why this part of philosophy was known as theoretical.

The experience appears to suggest as the first answer to the theoretical question that there are things that change in a number of ways. As we have seen, this viewpoint had been contested by Eleatics, who believed that there was only one existing thing and therefore nowhere that a change could lead to. Aristotle was quick to note that such a standpoint could not be properly handled in a science that assumes the existence of change. Still, he explained that Eleatic arguments could be understood as linguistic confusions. Thus, even if there would be only one existing entity, this entity could have a number of states, between which a change could happen. Furthermore, the Eleatics had at least described this supposed unitary entity with many terms: it was, for instance, either limited or unlimited. Such a description was then something different from the entity itself, and hence, in a sense even Eleatics had to accept multiplicity.

Because Eleatic arguments could then be ignored, we could accept the testimony of our senses and believe in the existence of change. Change, furthermore, seems to involve the existence of multiple states or things. A further question was then how many features one had to assume in order to account for change. Aristotle found Anaxagoras' theory of change insufficient. As we should remember, Anaxagoras had merely pointed out that e.g. a change of food to flesh could occur, because the constituents of flesh were among the constituents of food. Anaxagoras could then just say that constituents of everything were among constituents of everything else: as Aristotle pointed out, this explanation explained really nothing, because it didn't reveal what these constituents were.

Aristotle was then after a determinate number of features involved in change. His predecessors passed onto him two hints of these features. Firstly, many of earlier philosophers had noted that all change happened between two extreme states, like unity and multiplicity or hot and cold: indeed, all change moves from one state to another. Secondly, especially Platonists had assumed that the world of change was constituted by two things: a disordered chaos and an unchanging source of order regulating this chaos.

Now, Aristotle combined these two insights. The first extreme state of any change could be identified with some Platonic source or idea, or as Aristotle preferred to call it, form: a change either started or ended with a state of having some characteristic, such as the characteristic of being a doctor. The other extreme would then undoubtedly be a state of not having a characteristic. Platonic school had called this negative state matter, but Aristotle thought this was more of a confusion. Plato had not noticed a source of some characteristic could not affect a mere lack of itself (e.g. a mere lack of a doctor cannot become an existence of a doctor without anything to cause this generation of a doctor). Instead, change presupposed that something changed and this subject of change remained existent for the whole duration of this change: for instance, a person who is not doctor might become a doctor through proper education. This underlying thing that remained constant throughout any change could then be regarded as a third element in change, and indeed, this is what Aristotle preferred to call matter.

A certain type of change interested Aristotle partircularly, that is, changes that happened, when things were left to their own devices. Such changes could be called natural, and because Greek word for nature was physis, the study of these changes was called physics. Although the modern physics is a descendant of Aristotelian physics, there are certain clear differences. For instance, in Aristotle's physics it is the biological changes that are especially paradigmatic.

As we might remember from Aristotle's notion of sciences in general, any science should try to determine four aspects of the things it studies. Firstly, one ought to look for the conditions required for something: in case of changes one of these conditions was that what remained stable throughout the change or matter from which the result of the change was made. Secondly, one should try to find out what is the essence of what we are studying: in case of natural changes it is especially the essence of the result of the change we are interested in. Thirdly, one should try to ascertain what made the thing investigated: in this case, what initiated the change. Finally, one ought to find out what is the purpose of the investigated thing, and in this case, the purpose of the change. Now, in case of biological changes the aim of the change is often just to preserve the life of the individual or its genus: thus, the purpose and the essence of the change coincide. Furthermore, the initiating factor of the change is either the living being itself or an individual of the same kind. Hence, one could merely speak of two aspects in these cases – matter and essence or form of the biological change – and Aristotle assumes this is true of almost all natural changes.

Besides natural changes, the things of nature are often affected by chance occurrences. In such cases the occurrences themselves usually are natural, but they have unexpected consequences: for instance, it is natural that a stone falls down, but the falling stone may accidentally also hit a person's head and kill her. Aristotle noted that many earlier philosophers had believed all changes to be based on mere chance: for instance, Empedocles had suggested that in the past there had been generated accidentally all sorts of animals – bulls with human head, for instance – and that only some had been able to live and perpetuate their species. Aristotle noted that although nature did show such chance occurrences, clearly all the natural changes were somehow inevitable, unless some other circumstances prevented them from happening. Yet, such changes were not caused by some clockwork mechanism, because they clearly strived for some end, like perpetuation of species.

What then changes in general involve? According to Aristotle, all changes activated some capacities inherent in that which changed: when I start to build a bookshelf, the capacity of the planks and the nails to be reconfigured in some manner is actualised, and when a leaf starts to turn red, its capacity of being red is activated. Yet, in the state of change this activating process is not yet complete: a leaf turning to red is not yet completely red. Instead, the changing object has activated merely its capacity of being potentially something: a leaf turning to red is on its way to becoming red and is in a more essential sense potentially red than it was before. Changes thus differ from activities like seeing which has no other end except seeing itself: a change moves towards some perfection, which is distinct from the change and in which the change itself ends.

Many of the earliest philosophers had supposed that changes required some sort of limitless source from which new thing could proceed – otherwise, the changes would someday cease, when everything that could come to be would have come to be. Aristotle noted that we need not assume such limitless or infinite source of change, because changes could keep on happening between different possible states: e.g. a same object could constantly turn from red to green and from green to red. Aristotle was generally quite sceptical of any infinities. Certainly there were no infinite bodies, he insisted, because in an infinite universe we would have no meaningful way to differentiate between different directions and thus it would be senseless to say that e.g. heavy bodies move downwards. Then again, infinity was an acceptable concept, if it merely referred to some process being capable of indefinite continuation. Thus, time could be infinite in the sense that time could continue indefinitely, division of areas and lines could continue ”infinitely” or indefinitely long – although in a finite universe there could be only finite areas and lines – and no number of objects was the largest (although there was a smallest number).

Aristotle noted that changes could be classified in different manners. In some cases, what is said to change is not actually what changes, but it is only a part of that which changes or a whole, part of which is changing: thus, if a man is moving, his thoughts may be said to move with him, or when an eye is healing, we may say that the body with the eye is also healing. Yet, in all such cases there must be some primary thing that truly changes.

Now, in some cases, change involved a creation of a new entity or then a destruction of something: when I build a house, the house itself did not exist before the building. Aristotle noted that it was a controversial question whether any true creations or desrructions truly occurred, because in a sense, all changes must involve the continuation of existence: new existents (houses) are generated from previous existents (bricks) and not out of nothingness. Still, Aristotle thought, we can speak of generation and destruction when a substantial change happens, like when an elementary stuff is changed into another sort of elementary stuff. Thus, a generation of one substance involves always a destruction of another substance.

In most of the changes, nothing is created or destroyed, but some feature of an existing thing is altered. This feature might be a quality that is not essential to the thing in question – like when a leaf turns red. At other times even the shape of the thing might remain what it is, but the quantity of the thing might change into bigger or smaller – like when a balloon expands. In some cases no feature of the thing changes, but the thing merely moves from one place to another.

Aristotle was also sure that if we ignored creations and destructions, things could change in a primary sense only in these three manners: qualitatively, quantitatively or from one place to another. Things could still change incidentally in other manners: thus, when all childs of a person dies, the person also incidentally stopped being a parent, although no change had happened to her personally. Particularly, Aristotle thought that changes cannot be said to properly change or come to existence or to be destroyed: otherwise, all changes would involve an infinite number of other changes.

The possibility of moving from one place to another suggests the question what places in general are. Clearly a place is not a feature of the body in it: otherwise, another object could not take the place of another (like one leaf cannot take the colour of another leaf). Instead, the place of an object is, primarily, the innermost surface of things embracing the object in question: thus, the place of wine is the inner surface of the bottle where the wine is situated. Secondarily, the place of the object is then any place where any place of the object is situated: thus, because the bottle is in the fridge, the wine may be said to be in the fridge, and because the fridge is in the kitchen, the wine is also in the kitchen. The ultimate place in which everything then is situated is the outer fringe of the universe – remember that Aristotle argued for the finity of the universe, because otherwise we could not situate anything anywhere in a meaningful way.

Although Aristotle thus accepted the idea of an ultimate system of places or space, he was not convinced that space could exist without anything to fill it. Atomists had endorsed the idea of empty space, because they couldn't conceive how anything could move without there being something empty to which it could move. Aristotle noted that the movement could still happen, if the things would just change their places with one another. Furthermore, in empty space nothing could move, according to Aristotle: all motion required some push from other objects, but in an empty space nothing would push an object to move. On the other hand, in empty space all things would move with infinite speed: speed of a thing was, Aristotle suggested, proportional to the resistance of medium in which the movement happened, but in a void with no resistance, the speed would grow impossibly large. It is easy to laugh at such deductions, when we know better, but the Aristotelian notions of movement and space might seem more natural, if we knew nothing of inertia.

In addition to space, movement is also connected with time. Indeed, time appears to be essentially related to change. Even if we were in a dark cave and saw nothing happening, but something changed in our thoughts, we could be certain that time went forward. Time is then, Aristotle suggests, the act of counting out all changes and especially movement: the moment of ”now” moves through various changes and determines what happened before and after and at what rate. Although the counting might be something we humans do, time itself is in another sense not dependent of human beings, because the things counted or the temporal processes would occur even without human beings. Aristotle also suggests that a natural way to measure time would be to use some cyclical and recurring process, like the revolution of planets.

Aristotle was certain from experience that spatial magnitudes form a continuum: that is, e.g. volumes that are next to each other or that have nothing between them have limits that touch one another and that cannot be separated from one another. Aristotle concluded from this definition of spatial continuum that it could not be formed of indivisible points – if point would be continuous with another point, their limits would coincide and be inseparable, and because points have no other limits, but points themselves, the points would actually be identical with one another. Indeed, two points cannot be even immediately next to each other in space, because two different points will have some line between them. Aristotle noted also that if space was a continuum, motion through space must also be continuous (otherwise a moving object could instantly move from one area of space to another separate from the area it had occupied), if motion was continuous, so also the passage of time (because time measures all motions), and if passage of time was continuous, so finally all changes (because time measures also other changes).

Aristotle's idea of continuum was explicitly meant to show why Eleatic and especially Zeno's proofs against the existence of motion were invalid. Zeno had argued that an object couldn't move, because at any one moment it was in some place resting. Aristotle answered that time does not consist of moments, although it is limited by moments: when we divide a process, we find just smaller processes, but no indivisible unit of processuality. The so-called ”now” was then, according to Aristotle, no part of time, but a limit between past and future periods of time.

If Zeno's first argument targeted the idea of motion as consisting of indivisible moments, his other argument tried to combat the idea of continuous movement and time by noting that it involved a movement through an infinite number of points that would take infinitely time. Aristotle noted that although space, moved through by an object, did contain the possibility of being divided into smaller and smaller points with evermore limiting points, these limits did not really stop the motion for any time, unless the object stopped for a moment at such limit point: but the motion would take an infinite time only if the object stopped at every possible limiting point, because the smaller pieces of motion take up smaller periods of time.

We have already seen how Aristotle assumed the existence of changes. Furthermore, he was certain that changes had happened always and would always happen: indeed, because time was for Aristotle a measure of change, no time without change could exist. In addition, changes could not have begun from a generation of moving things, because all changes require something existent: even changes where new entities are formed, such as a construction of a chair, require some previous matter from which the new entity is made. Finally, time of change could not be preceded by a period of eternal rest of all entities: otherwise, there must have been some object hindering the possible changes, and the supposed first change would have to be preceded by the destruction of this hindering element.

Then again, Aristotle also was certain that not all things were changing at any current moment. Qualitative and quantitative changes would end at some time – becoming red when the changing thing had became red and growing at least when the growing thing reached the size of the finite universe. The only change that might continue forever would be motion, but experience appeared to confirm that not all things were moving: for instance, a stone might be restfully lying on the ground. Finally, some things apparently start changing, while others stop changing.

Aristotle also thought it reasonable to assume that all moving and changing things were moved or changed by something: either by some part of themselves or something separate from them. In case of forced changes this seems obvious, but unforced or natural changes seem a different case. Yet, even natural changes (like the fall of a heavy object) could be said to be caused by things that either changed the nature of the thing changing (e.g. the thing that solidified moisture in the clouds to snow that then would naturally fall down) or released the thing from any hindrances to its natural motion (like a person sawing table legs causes a table to fall down). Furthermore, even in the so-called self-moving entities, like animals, one could separate between the moving aspect and the moved aspect. Thus, all changes would be caused by things other than the things changing.

The next question would then be whether all moving things would be moved by some moving thing or whether there could be movements and changes instigated by something immovable: because Aristotle equated nature with the realm of moving and changing things, this immovable would have to be beyond nature. Aristotle approached the question by noting that a moving thing can be said to move another thing only if the original moving thing moves at the same time than the moved thing. Because the moving thing would require then another moving thing to move it, there would at the same limited time be an infinite number of moving things and thus an infinite amount of motion. As Aristotelian universe was limited, such an infinite motion during a limited time appeared an impossibility to him.

Although the consideration of such an unchanging cause of changes would indeed not belong in an investigation of changing things, one still must ponder what sort of changes this primary initiator would cause. Because this initiator does not itself change in any manner, it can initiate only a change that occurs constantly and goes on continuously. Clearly such change cannot be a generation of a new entity, destruction of an old entity, change from one quality to another or growth or diminution of an object, because such changes all have a final limit, beyond which they cannot continue. Neither can it be movement in a straight line, because in a finite universe, such a motion must eventually stop. The only remaining possibility then is that the primary initiator causes something to move around in a circle for eternity.

Most of the things Aristotle has discussed thus far are at the fringe of what nowadays is called physics, and indeed, might be classified as metaphysics.When Aristotle comes to more concrete matters, the more dated his discussions appear. Still, it is worth to look at the cosmology of Aristotle more carefully because of its long-lasting influence. Note that although later generations were to treat Aristotle's natural philosophy as a fully completed system, it was always more of a work in progress – thus, Aristotle knew that there were many open problems, answers to which were merely speculated by Aristotle and his followers.

We have seen that in Aristotelian universe of changing things, some things change sometimes, while others are in a continuous process of change, and more particularly, move in circles. Aristotle explains the difference between these two things by suggesting that the things moving in circles are made of different stuff than the other things, namely, from a stuff that naturally moves in circles. Because of the continuous nature of circular movement, this stuff seems more primary than other sorts of matter.

Aristotle feels also convinced that there is only one sort of matter moving naturally in circles: the circular movement defines this stuff. Still, there can be places in which this stuff is more concentrated than in others. Such spots appear to us as emitting light in comparison with the other heavenly regions. Aristotle also suggests in some passages that these spots are living and perhaps even conscious entities.

Aristotle also thinks that the world of the circularly moving stuff is divided into different layers: this is his attempt to explain the astronomical knowledge of his times. The outermost layer moves in a circle and the concentrated spots are carried by the layer: these spots are what we would call stars, which appear to move around the Earth in one day. This layer and its denizens are the most perfect moving things. Slightly lesser perfect are the spots moving in the inner laeyrs of the circularly moving stuff. The layers themselves move around the Earth, but the movement of the outer layers makes the spots also move within their own layers: this causes the apparently erratic movements of the planets. In the innermost layers the effect of the outer layers apparentyly twindles, because the layers moving the Sun and the Moon move again in a simpler manner. Aristotle also connects this structural difference of the layers with their corresponding levels of perfection: the stars in the outermost layer achieve the highest state of bliss through a simple movement, the planets require more effort to achieve this state and the Sun and the Moon cannot even achieve this level of perfection.

The circling movement of the heavenly stuff requires some fixed point around which it moves. This unmoving body should then be made of stuff that differs from the heavenly stuff. Indeed, the immobility of the central body or Earth is explained by the fact that the stuff which it is mostly made of – earth – moves naturally towards the centre of the universe: earth moves only when it is forcefully taken away from the center, and when it arrives at the centre, it stops moving.

The Earth differs then from stars and planets, because it allows the movement of bodies to stop. Indeed, the movements of earthly bodies seem to be of many different kinds. Aristotle now believed that this multiplicity of motions could be analysed through the motions of some simple sorts of stuff, which would then form the primary division of bodies in the centre of the universe. He also suggested that we should assume only a small number of such sorts of stuff or elements. Especially one should not think that there are infinitely many elements: the elements are to be differentiated by their characteristics, but there is only a limited number of elementary characteristics. Then again, one element is not sufficiecnt for explaining the existence of all the various phenomena in the world. Thus, we should choose a limited number of basic elements that would be differentiated by different characteristics.

We have already seen how Aristotle needs at least one sort of earthly stuff that moves towards the centre of the universe. Then again, he also points out that some things appear to move away from the centre, particularly fire that always move upwards. Between these two stuffs, earth moving always downwards and fire moving always upwards, there appears to be a stuff moving upwards, except when fire stops it – the air – and a stuff moving always downwards, except when earth stops it – water.

Aristotle also describes the relation of the four earthly stuffs or elements in terms of what he describes as basic qualities. The elements can be classified, firstly, according to the capacities of action that they have. Some things are hot, that is, they separate mixed combinations into their constituents and generally activate natural processes inherent in other things. This quality of ”hotness” appears to be associated with the upward movement: air and especially fire are hot. The opposite quality of ”coolness” – the capacity for combining various consituents and generally of hindering natural processes – is then linked to the downward movement of earth and water.

Aristotle also notes a second way to classify elements according to their capacities of receiving effects from other things. The extreme elements or fire and especially earth are dry in the sense that they have a definite shape, but are not easily malleable, while the mediating elements of air and water are moist in the sense that they have no definite shape, but are easily malleable.

Other qualities of elements and of their combinations should then be definable in terms of the four basic qualities. Thus, in a solid object the quality of dryness preponderates and similarly in a liquid object the quality of moistness preponderates.

It is the four primary qualities that actually define the basic elements, and what are usually called earth, water, air and fire are in fact only states of these elements. Particularly the hot and dry element is not always flaming, but could exist as a sort of dry gas, although it is the one most easily flammable: the meteors and comets Aristotle interprets as such inflammations of the hot and dry element.

Aristotle noted that the four earthly elements he had described do appear to change into one another: e.g. when water is applied to fire, a piece of earth is left behind, and when water is heated by fire, vapour or air is generated. In addition to such transformation, the elements can combine and so form more complex entities. The change of earthly things is thus accepted by Aristotle, but what originally instigated such changes? Aristotle's answer is that the earthly changes must eventually be caused by the movement of the heavens and particularly the closest spheres carrying the Sun and the Moon around the Earth.

By themselves, the four elements would in time move into their corresponding layers and remain stable for eternity. The movement of the spheres mixes the different layers, which thus form more like a continuum of different combinations of elements. For instance, atmosphere is formed of both dry and hot or ”fiery” element and moist and hot or ”aerious” element. When the dry air preponderates, the weather is windy, but when the moist air preponderates, clouds are formed and it begins to rain. Sometimes dry gas gets trapped in a cloud, and when it is released, it creates a loud noise called thunder and perhaps bursts into a flame called lightning. Similarly, earthquakes and volcanoes are explained by the dry gas being trapped within the Earth's crust.

The combination of the four basic elements produces then various natural substances. Aristotle never managed to give a detailed classification of all such combinations, but his examples suggest that the analysis of particular substances would have been based on the qualities of the substances in question. Thus, compounds of earth and water with earth in a predominating magnitude are dry and so become solid when heated, while compounds with water in predominating magnitude become solid when frozen. Neither of these things happens to olive oil, so Aristotle concludes that it is actually a combination of air and water.

The combination of elements leads first to substances that do not appear to consist of smaller parts, when the elements mix completely with one another. From such substances then more complex material structures are formed. The most intriguing composed things are undoubtedly living things, and indeed, Aristotle and his followers did spend much time in observing the compositions and behaviours of various plants and animals, thus beginning the sciences of botany and zoology. Aristotle even suggested some preliminary classifications of living things, for instance, he suggested that living things formed a hierarchy from less to more perfect. Yet, he offerred no complete classification and he was definitely against any a priori division of living things: one could not e.g. start in the Platonian manner from a notion of living being and by a constant division into two parts reach all the individual species.

Yet, even after observation and classification of living beings, it still remains uncertain what being alive actually means: what makes certain complex entities e.g. move about on their own accord? Aristotle did not believe that such liveliness would be caused by a separate element – material or immaterial – that could be used to fill bodies, like soup fills a bowl. Liveliness is not also, says Aristotle, identifiable with a single, harmonious state where elements would be mingled with one another in a certain proportion. In fact, the body would, according to this theory, stop being alive at once whenever a human being would fail to find this harmony.

Liveliness is then to be found in a process of living that structurises the passive materials into a living being. What is important in a living being is then the structure that the individual process of living tries to actualise. The materials used might be important in a secondary sense, namely, if the process of life requires certain type of material: for instance, if a living being creates a hard shell around its innards, it can use only hard enough substances for this purpose.

A living being, like a plant, has then a capacity to do certain things, such as nourishing itself when in contact with suitable material. The nourishment enters the living being through its superior part, which in the highest creature or man points towards the most superior parts of the world, and after the living being has appropriated the material it requires, it drops out the unrequired residue. Aristotle emphasises that it is the process of life that is active in the act of nourishment, while the nourishing material is just a means for rejuvenating the passive materials constituting the living entity.

Aristotle also connects the structure of life with his theory of material elements and basic physical qualities. Because heat means the capacity to activate natural processes, living beings as active must be warm rather than cold. But similarly living beings must be moist, because dry objects do not easily change their state, whereas living beings must do this. Thus, a living being must remain warm and moist, and a living entity which fails to retain its natural moistness and which secretes lot of liquid will probably not live very long.

The source of the heat activating the processes of life can then be regarded as the primary source of life. This source resides in the centre of the living thing, for instance, in the heart of a human being. The heat in animals together with their intestines reshapes the food in a form capable of nourishment (the plants do not require such reshaping, because they receive their nourishment in a form directly enjoyable). The final form of the nourishment in the more perfect animals is blood, which is by itself a cold substance, but as animated, acquires heat and then feeds and activates the whole body.

Although heat is central to living, a living being cannot live on mere heat, because mere heat by itself would finally quench itself, just like fire is finally extinguished when it has burned its fuel up. Thus, a living being must also control its heat and cool it in some manner. This is especially true of blooded animals, because animated blood is quite hot. In many blooded animals this happens through respiration, which uses air to cool the body: the exception are fishes that are cooled by the surrounding water. Another source of cooling in animals, according to Aristotle, is the brain, which is supposedly made of cold elements.

Beyond nourishing themselves, most living things can also produce other living beings and thus mimic the eternal movement of stars. Note that Aristotle thinks that not all living things are capable of reproducing. Some species that mix the characteristics of both plants and animals, like starfishes, are generated spontaneously from lifeless matter: a certain type of hot air is capable of instigating a process of life, e.g. in seas where all sorts of materials required for living are to be found. The results of such spontaneous generation cannot produce any offspring themselves – or at least they produce only imperfect offspring, incapable of any further generation.

Plants, according to Aristotle, generate individually, because of their simplicity. In the higher animals generation requires two individuals. One of them – the male – can develop its blood into a form that is capable of generating new living individual, when suitable material is given. This generating agent is semen, a substance generated out of blood and resembling oil in being a foamy mix of water and the forementioned hot air, but also with the capacity of becoming a living animal with sensations and motions.

The material for the semen is provided by the female, who is inferior, says Aristotle, and cannot produce any semen by herself. Instead, the female has an abundance of blood that can be enlivened by semen: if the blood is not used for producing a new living being, it will leak out in menstruation. Just like semen has the capacity of becoming a living animal, the blood in general has the capacity to become alive. After copulation, the semen activates the life of the blood in the woman and creates the central point or the heart, around which the new animal is generated. If the semen can completely control the material it uses, the resulting animal is a male of its kind, capable of producing new semen, and resembling its father. The loss of control means that the animal generated will fall short of perfection: it might be a woman, it might resemble an individual of earlier generation, or in worse case, it might completely fail to have the structure of its own kind.

Living things are not all of the same type, but form a hierarchy according to their capacities, and thus, different animals are generated in different manners. Very cold – that is, very passive – and very dry – that is, very inflexible – animals can only produce quite imperfect offspring that still require some development outside the generating animals: the offspring might be an egg that still grows before turning into a true animal, like with fishes, or even worse, a mere larva that still requires turning into an egglike cocoon before developing into an animal, like with insects. A more active, but still inflexible animal, like bird, can produce a hard egg, which needs not grow itself before turning into an animal; on the other hand, a more flexible, but still passive animal, like shark, can produce a soft and fragile egg, which the animal can turn in its own body into an animal. Finally, both active and flexible animals, like human beings, can produce full animals on their own.

In addition to methods of generation, Aristotle suggests various ways to classify living beings. For instance, plants live through earth, while higher animal life is based on air. Living beings that share characteristics of both animals and plants live in water, which is the element situated between air and earth. Aristotle also hints that Moon might be a home to life based on the fourth element, fire.

A more important differentiating factor is the capacity of sensation. While plants can only regenarate themselves and their species, animals have also the capacity to receive affections from their environment. That is, not to be affected by material things – plants are also affected in this sense – but merely by their outward structure. For instance, through vision an animal is affected in one way by visible or coloured things, in another way by an absence of visibility or darkness and in a third way by an overabundance of visibility or brightness. Usually the senses of the animals are situated at one side called front, which is also the direction to which the animals generally move.

Aristotle thinks that all sense affections require some mediating element, for instance, visible things are mediated by light and sounds and smells by air. Touch and taste Aristotle thinks to be peculiar in that the mediating element is actually a part of the body, that is, flesh and tongue. They are also the most primary forms of sensation, because all animals must be able to feel tangible things and to taste what they are eating. The flesh and the tongue both require some protection: some animals have a hard shell encasing the soft flesh, while others have hard bones keeping the flesh together, and the tongue is hidden behind lips and teeth.

Vision, hearing and smell, on the other hand, are required only by animals capable of movement, which need to sense things from far away. Smell is of these the closest to the contact senses, and indeed, smell and taste are closely connected. Just like tastes have a role in eating, smells vigorate, but only suitably developed animals. Colours, finally, are according to Aristotle the truest form of sensation there is: while e.g. speech of a person may be distorted by the air, the sensed colours should be at least close to how the object itself is coloured. Thus, when two colours form a new colour, the objects having these colours have also been combined into one object.

While some of the affections occur only in one sense, some, such as movement, are common to many senses. Thus, in addition to colours, sight can be used for sensing that a thing moves. In addition to such common sensibles, we may sense in a more figurative way more complex things, just like we can see an anger in a person, when we interpret him to be angry because of certain expressions we literally sense. Aristotle also thinks that the general capacity of sensation reveals not just characteristics of other objects, but also characteristics of ourselves. Thus, through vision we sense not just colour of apples and plumes, but also the fact that we are currently sensing apples and plumes. Aristotle also suggest that the central capacity of sensation lies at the same place as the source of the animal life, namely, at its heart.

When the general capacity to sense is temporarily nullified – e.g. through a certain phase in digestion, Aristotle suggests – the animal enters a state of sleep. Although the sensing proper has stopped, there might still be afterimages or dreams left from the daily events. An animal might even have a better view of his own bodily condition and dream about its possible ailments, because in a dream state the normally minimal signs of sickness are amplified.

In addition to sense, some animals have a capacity for, as it were, perceiving images of things that are not there. This capacity might be a cause of delusions, but it also allows the animals to consider possible outcomes of different processes. Furthermore, this capacity makes the animal capable of remembering previous events. Developed animals could even consciously regulate what they happen to remember through different mnemonic devices.

This capacity of imagining is closely connected with the ability of an animal to move itself for the sake of something. Indeed, without the capacity to remember and ponder possibilities, the animal could not do anything purposefully. This ability to move itself or act is still not completely independent, because it is instigated by something beyond the animal, namely, the desired conclusion of the action. The animal senses or imagines that something is the case and then instantly notes how to obtain what it wants. All this should happen at the centre or the heart of the animal, and from there the impulse to change the state of the body begins. Particularly the impulse to move goes from heart to the joints that move the limbs with the aid of immovable earth. Just as nourishment differentiates between the superior and the inferior parts and sensation between the front and the back, the movement supposedly differentiates between the right and the left: animals begin the movement with their right side.

The highest capacity of a living being is reserved, according to Aristotle, to the highest form of living being, that is, humans: Aristotle even hints that this capacity is transplanted into a generated human being from a source external to the parents. Humans have the capacity to consider not just sensed characteristics, but all sorts of structures in their thinking. Thinking person is then just like a universal capacity for considering all possible things there is to consider. This capacity also gives humans more powers in their deliberation over what to do: they can reason what is truly worth a deed. Beyond this passive capacity, Aristotle suggests, there exists an activity that arouses different structures for the passive capacity to think. This activity, Aristotle thinks, is not anymore capable of change, but exists in a higher realm than mere physical world of variability.

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