These chapters, LIX-LXXVIII, are the most abtruse in the whole work. They are founded on the scholastic theory of the origin of ideas, which again is based on Aristotle, De anima, III, Chapp. IV, V. The theory first presupposes the doctrine of matter and form, of which there is a fair]y good account in Grote's Aristotle, vol. II, pp. 181-196. Grote goes on to expose the Aristotelian doctrine of Nous (intellectus), as he understands it. In this exposition two points are noteworthy. (i) No account is taken of St Thomas's distinction between potential (possibilis) and 'passive' (passivus) intellect. (2) A view is ascribed to Aristotle, closely allied to the views which Averroes and Avicenna ascribe to him, views which St Thomas laboriously combats as being neither Aristotelian nor correct. If these Mohammedan commentators, with Grote and many moderns, are right, Aristotle cannot be claimed as a believer in personal immortality. Still the fact that Plato steadily held the individual soul to be immortal, joined to the fact that Aristotle, who was forward enough in contradicting his master, nowhere explicitly contradicts him on this head, -- as also the obscurity of the language of the De anima, -- "may give us pause."
For any understanding of what follows it is necessary to distinguish the 'passive intellect' (intellectus passivus, nous pathêtikos), the 'potential intellect' (intellectus possibilis, nous dunatos, or ho dunamei nous), and the 'active intellect' (intellectus agens, nous poiêtikos).
1. 'Passive intellect' is not intellect at all. It is found in the higher dumb animals; and is only called 'intellect' by a sort of brevet rank, because being the highest power of the sensitive soul, it comes closest to intellect and ministers to it most nearly. St Thomas calls it in dumb animals vis aestimativa; in man, vis cognativa and ratio particularis. It has no English name, but may be defined: 'an instinct whereby the sentient soul directly recognises a sensible object as a particular something here and now present.' See Father Bödder's Psychologia, pp. 71-79, who apposite]y cites Cardinal Newman's Grammar of Assent, pp. 107 sq. See too Silvester Maurus, Commentary on Aristotle, De anima, lib. III, cap. iv (ed Lethielleux, Paris, 1886, tom. IV, pp. 94, 95) Aristotle tells us of this faculty that it perishes with the body, but that its operation is an indispensable preliminary to all human understanding, ho de pathêtikos nous phthartos, kai aneu toutou outhen noei (De anima III, v, ult.)
2. Much more important is the 'potential intellect,' -- intellectus possibilis, a term occurring again and again in all the writings of the schoolmen, being founded on one word of Aristotle, De anima III, iv, 3, med autou einai phusin oudemian all ê tautên hoti dunaton (nor has it any other natural property than this, that it is able, capable, potential). It is defined by Maurus (l.c.): "the intellect inasmuch as it is capable of being [representatively] made all things, by receiving intelligible impressions of all things." An 'intelligible impression' differs from a 'sensible impression' as the universal from the particular, e.g. as the triangle in the mind, which stands for any triangle, from the image of this particular triangle chalked on the board and taken up by sense and phantasy.
3. Of equal scholastic importance is the 'active intellect,' intellectus agens, defined by Maurus: "The intellect inasmuch as it is capable of [representatively] making all things, by impressing on the potential intellect intelligible impressions of all things." The term nous poiêtikos though not actually found, is implied in De anima, III, v. The 'active' and 'potential' intellect together make up the understanding. The exact extent of the distinction between them is matter of some dispute (Bödder, Psychologia, pp. 159-163).
What ordinary mortals call 'intellect' or 'understanding,' is the 'potential intellect.' It is called 'potential' because it is open to all intellectual impressions, and, prior to experience, is void of all impression, and has no predisposition of itself to one impression rather than to another. This by the way seems to militate against the Kantian doctrine of intellectual 'categories,' or 'forms of mind.' But it does not militate against the doctrine of heredity. Heredity works in the body, in the domain of the sentient soul: we are here concerned with pure intellect. Of that, Aristotle says it is "impassible [i.e., not directly acted on by matter], yet apt to receive the intelligible impression, or form; but has no formed impression upon it, before the process of understanding is set up." The 'active intellect' on the other hand is the act of spontaneous energy, whereby the intellect transforms the image, sent up to it by sense and phantasy, from particular to universal, making out of it an 'intelligible impression.' A further distinction is drawn between the 'intelligible impression' (species intelligibilis impressa) thus created and received in the mind, and the 'intelligible expression' (species intelligibilis expressa), or precise act whereby the mind understands. See Bödder, Psychologia, pp. i 53-i 56. This distinction has been already drawn by St Thomas (B. I, Chap. LIII).
For further elucidation see Father Maher's Psychology, pp. 3O4-313, ed. 4, who however speaks of intellectus patiens vel possibilis, and takes no account of the intellectus passivus of St Thomas (B. II, Chap. LX), probably because it simply is not intellect.
Of God and His Creatures: 2.59