Difference may be predicated commonly, properly, and most properly: for one thing is said to differ from another in common from its differing in some respect in diversity of nature, either from itself, or from something else; for Socrates differs from Plato in diversity of nature, and himself from himself when a boy, and when become a man, also when he does any thing, or ceases to do it, and it is always perceived in the different ways in which a thing is somehow effected. Again, one thing is said to differ properly from another, when one differs from another by an inseparable accident; but an inseparable accident is such as blueness, or crookedness, or a scar become scirrhous from a wound. Moreover, one is most properly said to differ from another, when it varies by specific difference, as man differs from horse by specific difference, i. e. by the quality of rational. Universally then every difference acceding to a thing renders it different, but differences common and proper render it different in quality, and the most proper render it another thing. Hence, those which render it another thing are called specific, but those,  which make it different in quality, are simply (called) differences, for the difference of rational being added to animal, makes it another thing, (and makes a species of animal,) but difference of being moved makes it different in quality only from what is at rest, so that the one renders it another thing, but the other only of another quality.

According then, to the differences which produce another thing do the divisions of genera into species arise, and the definitions arising from genus and such differences are assigned. On the other hand, as to those which only make a thing different in quality, diversities alone consist, and the changes of subsistence of a thing; beginning then, again, from the first, we must say that of differences some are separable, others inseparable, thus to be moved, and to be at rest, to be ill, and to be well, and such as resemble these, are separable, but to have a crooked, or a flat nose, to be rational, or irrational, are inseparable differences. Again, of the inseparable, some exist per se, others by accident, for rational, mortal, to be susceptible of science, are inherent in man per se, but to have a crooked or flat nose, accidentally, and not per se. Wherefore, such as are present per se, are assumed in the definition of substance, and effect a different thing, but what are accidental arc neither taken in the definition of substance, nor render a thing another, but of another quality. Those too, which are per se, do not admit of the more and less, but the accidental, even if they be inseparable, admit of intention and remission,  for neither is genus more and less predicated of that of which it is the genus, nor the differences of genus according to which it is divided. For these are such as complete the definition of each thing, but the essence of each is one and the same, and neither admits of intention, nor remission; to have however a crooked or a flat nose, or to be in some way coloured, admits both of intension and remission. Since then, there are three species of difference considered, some indeed separable, but others inseparable, again, of the inseparable, some are per se, but others accidental, moreover of differences per se, some are those according to which we divide genera into species, but others according to which the things divided become specific:--thus of all such differences per se of animal as these, animated and sensitive, rational and irrational, mortal and. immortal, the difference of animated and sensitive is constitutive of the essence of animal, for animal is an animated substance, endued with sense, but the difference of mortal and immortal, and that of rational and irrational, are the divisive differences of animal, for through these we divide genera into species: yet these very differences which divide the genera are constitutive and completive of species. For animal is divided by the difference of rational and irrational, and again, by the difference of mortal and immortal; but the differences of rational and mortal are constitutive of man, but those of rational and immortal of God, those again, of mortal and irrational, of irrational animals. Thus also, since the differences of animate and inanimate, sensitive and void of sense, divide the highest substance, animate and sensitive added to substance, complete animal, but animate and deprived of sense, form plant; since then, the same   differences taken in one way become constitutive, but in another divisive, they are all called specific.

These indeed are especially useful for divisions of genera, and for definitions, yet not with regard to those which are inseparable accidentally, nor still more with such as are separable. And indeed defining these, they say that difference is that by which species exceeds genus, e. g. man exceeds animal in being rational and mortal, for animal is neither any one of these, (since whence would species have differences?) nor has it all the opposite differences, (since otherwise the same thing would at the same time have opposites,) but (as they allege) it contains all the differences which are under it in capacity, but not one of them in energy, and so neither is any thing produced from non-entities, nor will opposites at the same time subsist about the same thing.

Again, they define it (difference) also thus: difference is that which is predicated of many things differing in species in answer to the question, of what kind a thing is, for rational and mortal being predicated of man, are spoken in reply to what kind of thing man is, and not as to the question what is he. For when we are asked what is man, we properly answer, an animal, but when men inquire what kind of animal, we say properly, that he is rational and mortal. For since things consist of matter and form, or have a constitution analogous to matter and form, as a statue is composed of brass, matter, but of figure, form, so also man, both common and specific, consists of matter analogous to genus, and of form analogous to difference, but the whole of this, animal, rational, mortal, is  man, in the same manner as the statue there. They also describe it thus, difference is what is naturally adapted to separate things which are under the same genus, as rational and irrational separate man and horse, which are under the same genus, animal. Again, they give it in this way: difference is that by which each singular thing differs, for man and horse do not differ as to genus, for both we and horses are animals, but the addition of rational separates us from them; again, both we and the gods  are rational, but the addition of mortal separates us from them. They however who more nicely discuss what pertains to difference, say that it is not any casual thing dividing those under the same genus, but such as contributes to the essence, and to the definition of the essence of a thing, and which is part of the thing. For to be naturally adapted to sail is not the difference, though it is the property of man, since we may say that of animals, some are naturally adapted to sail, but others not, separating man from other animals; yet a natural ability to sail does not complete the essence, neither is a part of it, but only an aptitude of it, because it is not such a difference as those which are called specific differences. Wherefore specific differences will be such as produce another species, and which are assumed in explaining the very nature of a thing: and concerning difference this is sufficient.