De Anima

The process of thinking indivisible wholes belongs to a sphere from which falsehood is excluded. But where both truth and falsehood are possible there is already some combining of notions into one. As, in the words of Empedocles, "where sprang into being the neckless heads of many creatures," then afterwards Love put them together, so these notions, first separate, are combined; as, for instance, the notions incommensurable and diagonal. And, if the thinking refers to the past or to the future, the notion of time is included in the combination. Falsehood, in fact, never arises except when notions are combined. For, even if white be asserted to be not-white, not-white is brought into a combination. We may equally well call every statement a disjunction. But at any rate under truth and falsehood we include not only the assertion that Cleon is white, but also the assertion that he was or will be. And the unifying principle is in every case the mind.

Since, however, the term indivisible has two meanings, according as a whole is not potentially divisible or is actually undivided, there is nothing to hinder us from thinking an indivisible whole, when we think of a length (that being actually undivided), or from thinking it in an indivisible time. For the time is a divisible or indivisible unit in the same way as the length thought of. We cannot therefore state what the mind thinks in each half of the time. For, if the whole be undivided, the half has only potential existence. But, if the mind thinks each half separately, it simultaneously divides the time also. And in that case it is as if the parts were separate lengths. If, however, the mind conceives the length as made up of the two halves, then the time may be regarded as made up of corresponding halves.

Again, that which is not quantitatively but specifically an indivisible whole the mind thinks in an indivisible unit of time and by an indivisible mental act. Per accidens, however, such specific unity is divisible, though not in the same way as they, the act of thought and the time required for the act, are divisible, but in the same way as they are whole and indivisible. For in these specific unities also there is present a something indivisible, though certainly not separately existent, the same as that which constitutes the unity of both the time and the length. And, as with time and length, so in like manner with whatever is continuous. But the point and every division and whatever is an undivided whole in the same sense as the point is clearly explained by the analogy of privation. And the same explanation holds in all other cases. How, for instance, is evil apprehended, or black? In some fashion by its contrary. But that which apprehends must potentially be, and must contain within itself, the contrary which it apprehends. If, however, there be something which has no contrary [some one of the causes], then it is itself the content of its own knowledge, is in actuality and is separately existent.

Now every proposition, like an affirmative proposition, predicating something of something, is true or false. But with thought this is not always so. When its object is the What in the sense of the quiddity and there is no predication, thought is in every case true. But, as the perception by sight of the proper object of sight is infallibly true, whereas in the question whether the white object is a man or not, perception by sight is not always true, so is it with immaterial objects.