Some philosophers think that the last agent-the agent in the strictest sense-enters in through certain pores, and so the patient suffers action. It is in this way, they assert, that we see and hear and exercise all our other senses. Moreover, according to them, things are seen through air and water and other transparent bodies, because such bodies possess pores, invisible indeed owing to their minuteness, but close-set and arranged in rows: and the more transparent the body, the more frequent and serial they suppose its pores to be. Such was the theory which some philosophers (including Empedocles) advanced in regard to the structure of certain bodies. They do not restrict it to the bodies which act and suffer action: but combination too, they say, takes place only between bodies whose pores are in reciprocal symmetry. The most systematic and consistent theory, however, and one that applied to all bodies, was advanced by Leucippus and Democritus: and, in maintaining it, they took as their starting-point what naturally comes first.
For some of the older philosophers thought that what is must of necessity be one and immovable. The void, they argue, is not: but unless there is a void with a separate being of its own, what is cannot be moved-nor again can it be many, since there is nothing to keep things apart. And in this respect, they insist, the view that the universe is not continuous but discretes-in-contact is no better than the view that there are many (and not one) and a void. For (suppose that the universe is discretes-in-contact. Then), if it is divisible through and through, there is no one, and therefore no many either, but the Whole is void; while to maintain that it is divisible at some points, but not at others, looks like an arbitrary fiction. For up to what limit is it divisible? And for what reason is part of the Whole indivisible, i.e. a plenum, and part divided? Further, they maintain, it is equally necessary to deny the existence of motion.
Reasoning in this way, therefore, they were led to transcend sense-perception, and to disregard it on the ground that one ought to follow the argument: and so they assert that the universe is one and immovable. Some of them add that it is infinite, since the limit (if it had one) would be a limit against the void.
There were, then, certain thinkers who, for the reasons we have stated, enunciated views of this kind as their theory of The Truth.... Moreover, although these opinions appear to follow logically in a dialectical discussion, yet to believe them seems next door to madness when one considers the facts. For indeed no lunatic seems to be so far out of his senses as to suppose that fire and ice are one: it is only between what is right and what seems right from habit, that some people are mad enough to see no difference.
Leucippus, however, thought he had a theory which harmonized with sense-perception and would not abolish either coming-to-be and passing-away or motion and the multiplicity of things. He made these concessions to the facts of perception: on the other hand, he conceded to the Monists that there could be no motion without a void. The result is a theory which he states as follows: The void is a not being, and no part of what is is a not-being; for what is in the strict sense of the term is an absolute plenum. This plenum, however, is not one: on the contrary, it is a many infinite in number and invisible owing to the minuteness of their bulk. The many move in the void (for there is a void): and by coming together they produce coming to-be, while by separating they produce passing-away. Moreover, they act and suffer action wherever they chance to be in contact (for there they are not one), and they generate by being put together and becoming intertwined. From the genuinely-one, on the other hand, there never could have come-to-be a multiplicity, nor from the genuinely-many a one: that is impossible. But (just as Empedocles and some of the other philosophers say that things suffer action through their pores, so) all alteration and all passion take place in the way that has been explained: breaking-up (i.e. passing-away) is effected by means of the void, and so too is growth-solids creeping in to fill the void places. Empedocles too is practically bound to adopt the same theory as Leucippus. For he must say that there are certain solids which, however, are indivisible-unless there are continuous pores all through the body. But this last alternative is impossible: for then there will be nothing solid in the body (nothing beside the pores) but all of it will be void. It is necessary, therefore, for his contiguous discretes to be indivisible, while the intervals between them-which he calls pores-must be void. But this is precisely Leucippus theory of action and passion.
Such, approximately, are the current explanations of the manner in which some things act while others suffer action. And as regards the Atomists, it is not only clear what their explanation is: it is also obvious that it follows with tolerable consistency from the assumptions they employ. But there is less obvious consistency in the explanation offered by the other thinkers. It is not clear, for instance, how, on the theory of Empedocles, there is to be passing-away as well as alteration. For the primary bodies of the Atomists-the primary constituents of which bodies are composed, and the ultimate elements into which they are dissolved-are indivisible, differing from one another only in figure. In the philosophy of Empedocles, on the other hand, it is evident that all the other bodies down to the elements have their coming-to-be and their passing-away: but it is not clear how the elements themselves, severally in their aggregated masses, come-to-be and pass-away. Nor is it possible for Empedocles to explain how they do so, since he does not assert that Fire too (and similarly every one of his other elements) possesses elementary constituents of itself.
Such an assertion would commit him to doctrines like those which Plato has set forth in the Timaeus. For although both Plato and Leucippus postulate elementary constituents that are indivisible and distinctively characterized by figures, there is this great difference between the two theories: the indivisibles of Leucippus (i) are solids, while those of Plato are planes, and (ii) are characterized by an infinite variety of figures, while the characterizing figures employed by Plato are limited in number. Thus the comings-to-be and the dissociations result from the indivisibles (a) according to Leucippus through the void and through contact (for it is at the point of contact that each of the composite bodies is divisible), but (b) according to Plato in virtue of contact alone, since he denies there is a void.
Now we have discussed indivisible planes in the preceding treatise. But with regard to the assumption of indivisible solids, although we must not now enter upon a detailed study of its consequences, the following criticisms fall within the compass of a short digression: i. The Atomists are committed to the view that every indivisible is incapable alike of receiving a sensible property (for nothing can suffer action except through the void) and of producing one-no indivisible can be, e.g. either hard or cold. Yet it is surely a paradox that an exception is made of the hot-the hot being assigned as peculiar to the spherical figure: for, that being so, its contrary also (the cold) is bound to belong to another of the figures. If, however, these properties (heat and cold) do belong to the indivisibles, it is a further paradox that they should not possess heaviness and lightness, and hardness and softness. And yet Democritus says the more any indivisible exceeds, the heavier it is-to which we must clearly add and the hotter it is. But if that is their character, it is impossible they should not be affected by one another: the slightly-hot indivisible, e.g. will inevitably suffer action from one which far exceeds it in heat. Again, if any indivisible is hard, there must also be one which is soft: but the soft derives its very name from the fact that it suffers a certain action-for soft is that which yields to pressure.
II. But further, not only is it paradoxical (i) that no property except figure should belong to the indivisibles: it is also paradoxical (ii) that, if other properties do belong to them, one only of these additional properties should attach to each-e.g. that this indivisible should be cold and that indivisible hot. For, on that supposition, their substance would not even be uniform. And it is equally impossible (iii) that more than one of these additional properties should belong to the single indivisible. For, being indivisible, it will possess these properties in the same point-so that, if it suffers action by being chilled, it will also, qua chilled, act or suffer action in some other way. And the same line of argument applies to all the other properties too: for the difficulty we have just raised confronts, as a necessary consequence, all who advocate indivisibles (whether solids or planes), since their indivisibles cannot become either rarer or derser inasmuch as there is no void in them.
III. It is a further paradox that there should be small indivisibles, but not large ones. For it is natural enough, from the ordinary point of view, that the larger bodies should be more liable to fracture than the small ones, since they (viz. the large bodies) are easily broken up because they collide with many other bodies. But why should indivisibility as such be the property of small, rather than of large, bodies?
IV. Again, is the substance of all those solids uniform, or do they fall into sets which differ from one another-as if, e.g. some of them, in their aggregated bulk, were fiery, others earthy? For (i) if all of them are uniform in substance, what is it that separated one from another? Or why, when they come into contact, do they not coalesce into one, as drops of water run together when drop touches drop (for the two cases are precisely parallel)? On the other hand (ii) if they fall into differing sets, how are these characterized? It is clear, too, that these, rather than the figures, ought to be postulated as original reals, i.e. causes from which the phenomena result. Moreover, if they differed in substance, they would both act and suffer action on coming into reciprocal contact.
V. Again, what is it which sets them moving? For if their mover is other than themselves, they are such as to suffer action. If, on the other hand, each of them sets itself in motion, either (a) it will be divisible (imparting motion qua this, being moved qua that), or (b) contrary properties will attach to it in the same respect--i.e. matter will be identical in-potentiality as well as numerically-identical.
As to the thinkers who explain modification of property through the movement facilitated by the pores, if this is supposed to occur notwithstanding the fact that the pores are filled, their postulate of pores is superfluous. For if the whole body suffers action under these conditions, it would suffer action in the same way even if it had no pores but were just its own continuous self. Moreover, how can their account of vision through a medium be correct? It is impossible for (the visual ray) to penetrate the transparent bodies at their contacts; and impossible for it to pass through their pores if every pore be full. For how will that differ from having no pores at all? The body will be uniformly full throughout. But, further, even if these passages, though they must contain bodies, are void, the same consequence will follow once more. And if they are too minute to admit any body, it is absurd to suppose there is a minute void and yet to deny the existence of a big one (no matter how small the big may be), or to imagine the void means anything else than a bodys place-whence it clearly follows that to every body there will correspond a void of equal cubic capacity.
As a general criticism we must urge that to postulate pores is superfluous. For if the agent produces no effect by touching the patient, neither will it produce any by passing through its pores. On the other hand, if it acts by contact, then-even without pores-some things will suffer action and others will act, provided they are by nature adapted for reciprocal action and passion. Our arguments have shown that it is either false or futile to advocate pores in the sense in which some thinkers conceive them. But since bodies are divisible through and through, the postulate of pores is ridiculous: for, qua divisible, a body can fall into separate parts.