Appendix - 3. (223).

My apparent departure here from the opinion of St. Thomas (who upholds the Aristotelian definition of the soul as an act of the body) should not cause surprise. In taking as our teachers all the holy doctors of the Church, it is I think a duty of lovers of truth to cling to the basic principles rather than the letter of their teaching. There are sometimes contradictions in the letter which are absent in the principles. In his time, St. Thomas was almost obliged to hold the teaching of Aristotle or correct it with caution. As far as I can see, Aristotle's definition of the soul cannot be upheld. Calling the soul act of the body appears to render it a production of the body; acts are produced by their subject. The image he uses of wax and a shape impressed on it (De Anima, bk. 2) shows that this criticism is correct. In fact, the wax in which the impression has been made is one substance, wax; the image is but an act, a simple modification of the wax. Thus, the soul would not be a substance, but a simple modification of the body.

St. Thomas was aware of this, and although he retained the Aristotelian definition, he was not prepared to accept its consequent error. In his own objection about the possibility of the intellect's being an act of the body, he replied that 'the human soul is an act of the organic body to the extent that the body provides an organ for it. Nevertheless, the body need not be the organ of the soul relative to every potency and power because the soul exceeds the proportion of the body' (De An., bk. 2, ad 2). One part of the soul, the principal part, is not therefore an act of the body. Consequently, the Aristotelian definition does not express the essence of soul as a good definition should; if the essence were nothing more than an act of the body, the intellect would not be soul, nor pertain to the essence of soul because it makes no use of any bodily organ.

This is an invincible objection which certainly did not escape the insight of St. Thomas. He did not resolve the difficulty, but was content to safeguard the more important truth by replying simply: 'The possible intellect is consequent to the concept of soul in so far as the soul exceeds bodily matter. Hence although it is not the act of any organ, the intellect is not entirely outside the essence of the soul, but is that which is supreme in it' (Q. De An., art. 2, ad 4). In abandoning Aristotle's definition, I think therefore that substantially I am upholding Aquinas' teaching. This is all the more necessary when we consider that intelligence is not a mere potency of the human soul, but a part of its essence, a substantial and specific part.