Avicenna's theory tends to make the active intellect from without supply the potential intellect with intelligible forms: in which case phantasms cease to be necessary as a previous condition for the acquisition of intellectual ideas; and the arguments in the last chapter, which suppose such necessity of phantasms, fall to the ground. Averroes supposed one universal intellect of all men, at once potential and active: he left the individual, merely as such, nothing higher than the sentient powers. Avicenna denied to the individual the active intellect, and supposed one universal active intellect for all mankind. The potential intellect is reduced by his theory to a momentary impressibility.
Avicenna (Abu Ali Ibn-Sina), a native of Persia, lived A.D. 980-1037. Like Averroes, he was physician and philosopher. I quote from The Psychology of Ibn-Sina translated by J. M. Macdonald, M.A., Beyruth 1884. Four faculties are distinguished by Avicenna all of them belonging to the sentient part of the soul: none of them to the intelligent part. They are called "conceptual faculty," "imagination," "judgement," "memory."
I. Conceptual faculty. "There is nothing in the conceptual faculty besides the true forms derived from sense" (p. 28). This seems to correspond to what St Thomas calls virtus apprehensiva sensibilis, the faculty of sense perception.
II. Imagination. "In animals there is a faculty which compounds whatever forms have been collected in the common sense, and distinguishes between them, and differentiates them, without the disappearance of the forms from common sense; and this faculty is named imagination" (p. 28). "The imaginative faculty performs its actions without perceiving that things are according to its imaginings" (p. 28). "The imaginative faculty may imagine things other than that which the judgement considers desirable" (p. 29). If we might assume that this 'imagination' is purely reproductive of sense phantasms, it would answer to the 'phantasy' (imaginatio) which St Thomas ascribes to Avicenna.
III. Judgement. "Then in animals there is a faculty which decides decisively upon a thing, whether it is this or not. And by it the animal flies from that which is to be guarded against, and seeks that which is desirable. This faculty is called the judging and the supposing faculty" (pp. 28, 29). It is not difficult to recognise here that highest faculty of animal nature, called in other animals vis aestimativa, in man vis cogitativa (Chap. LX).
IV. Memory. "Then there is in animals a faculty which preserves the meaning of that which the faculties have conceived, e.g., that the wolf is an enemy." It is a store-house of judgements rather than of sense perceptions: for "the senses do not perceive the enmity of the wolf; or the love of the child": only the vis aestimativa perceives that, "then it treasures them up in this faculty." It is not a store-house of fancies, as the "imagination" is: for "this faculty does not picture anything which the judgement does not approve. This faculty does not declare anything to be true, but preserves what something else declares to be true. And this faculty is called the preserving and remembering faculty" (p. 29). All this answers exactly to the account of "memory" which St Thomas attributes to Avicenna.
We come now to the main argument of this chapter, which is Avicenna's belief in the 'active intellect' as a separate intelligence, working causatively upon the mind of man, and generating therein universal concepts, such concepts not being stored in the human mind for future use, but directly created afresh for every recurrence of them, by the action of this extrinsic intelligence. Against this doctrine of Avicenna, Averroes writes explicitly (De animae beatitudine cap. iii, p. 151): Intellectus agens non tantum est causa in intellectu materiali [sc. possibili] per viam efficientis et motoris, sed per viam ultimae perfectionis, hoc est, per viam formae et finis. (See note, p. 135.) Averroes united the active and the potential intellect, and made both eternal: Avicenna and Alexander made the active intellect alone eternal. Avicenna's theory of the universal active intellect is thus given in his own quaint words. -- " The proving of the existence of an intellectual essence, distinct from bodies, standing in the relation of light to sight, and in the place of a fountain: and the proving that, when human souls separate from bodies, they unite with this essence" (Title of Section x, p. 40). Speaking of the belief in mathematical axioms, he says: "It must be either by the use of sense and experiment, or by divine continuous overflow, overflow continuous with the rational soul, and the rational soul continuous with it. . . . This overflow, which is continuous with the soul, is an intellectual essence, not a body, not in a body: it stands by itself, holding the relation to the intellectual soul of light to sight" (pp. 40, 41). "The soul remains after death ever immortal, joined on to this noble essence, which is universal intelligence" (p. 42).
In Avicenna, as in Averroes, one recognises in the doctrine of ittisál however misdirected, that craving for some connexion of man's intelligence with a spirit above his own, which a banal materialism or positivism labours to extirpate, making man highest of beings and (perforce) self-sufficient. That craving is the root of mysticism; and in the doctrine of the Incarnation, with its corollaries of grace and sacraments, it has become the animating principle of Christianity.
Of God and His Creatures: 2.74