Let explain the way in which things in fact possess the power of generating, and of acting and suffering action: and let us start from the principle we have often enunciated. For, assuming the distinction between (a) that which is potentially and (b) that which is actually such-and-such, it is the nature of the first, precisely in so far as it is what it is, to suffer action through and through, not merely to be susceptible in some parts while insusceptible in others. But its susceptibility varies in degree, according as it is more or less; such-and such, and one would be more justified in speaking of pores in this connexion: for instance, in the metals there are veins of the susceptible stretching continuously through the substance.
So long, indeed, as any body is naturally coherent and one, it is insusceptible. So, too, bodies are insusceptible so long as they are not in contact either with one another or with other bodies which are by nature such as to act and suffer action. (To illustrate my meaning: Fire heats not only when in contact, but also from a distance. For the fire heats the air, and the air-being by nature such as both to act and suffer action-heats the body.) But the supposition that a body is susceptible in some parts, but insusceptible in others (is only possible for those who hold an erroneous view concerning the divisibility of magnitudes. For us) the following account results from the distinctions we established at the beginning. For (i) if magnitudes are not divisible through and through-if, on the contrary, there are indivisible solids or planes-then indeed no body would be susceptible through and through :but neither would any be continuous. Since, however, (ii) this is false, i.e. since every body is divisible, there is no difference between having been divided into parts which remain in contact and being divisible. For if a body can be separated at the contacts (as some thinkers express it), then, even though it has not yet been divided, it will be in a state of dividedness--since, as it can be divided, nothing inconceivable results. And (iii) the suposition is open to this general objection--it is a paradox that passion should occur in this manner only, viz. by the bodies being split. For this theory abolishes alteration: but we see the same body liquid at one time and solid at another, without losing its continuity. It has suffered this change not by division and composition, nor yet by turning and intercontact as Democritus asserts; for it has passed from the liquid to the solid state without any change of grouping or position in the constituents of its substance. Nor are there contained within it those hard (i.e. congealed) particles indivisible in their bulk: on the contrary, it is liquid--and again, solid and congealed--uniformly all through. This theory, it must be added, makes growth and diminution impossible also. For if there is to be opposition (instead of the growing thing having changed as a whole, either by the admixture of something or by its own transformation), increase of size will not have resulted in any and every part.
So much, then, to establish that things generate and are generated, act and suffer action, reciprocally; and to distinguish the way in which these processes can occur from the (impossible) way in which some thinkers say they occur.