A very intriguing textual question has arisen recently concerning St. Thomas' commentary on Aristotle's De Anima. I invite comments and clarifications, etc. This is somewhat tied to the inchoate stream of thought that is currently my term paper.

First I will give the context: St. Thomas, in this opening chapter, is expositing Aristotle's prologue. He shows that first Aristotle wishes to render his reader benevolent towards learning this science of psychology by showing the worth of this knowledge. This he does in two ways: first by showing that the science is dignified, i.e. that its object is honorable, and second, because its knowledge is useful.

While this in itself is interesting, that a speculative science is useful, St. Thomas names three sciences for which psychology is 'utile.'

First, it is useful to first philosophy. More on this in a bit. Second, it is useful for ethics because it outlines the powers of soul for which there are various virtues, treated of in the ethical science. Third, it is useful for physics, because it explains the causes of motions of living things, which make up the bulk of mobile being.

However, concerning this first reason, there are two quandaries. The first is on the level of littera, the second that of sensus.

On the level of littera; I quote first from the Leonine edition of the text (Leon. v.41, t. 1; p. 5-6; lines 118-122):

"Quia si ad philosophiam primam attendamus, non possumus deuenire in cognitionem divinorum et altissimarum causarum nisi per ea que ex uirtute intellectus primo acquirimus. Si uero attendatur quantum ad moralem..."

Compare with the text from the Marietti edition, which is also used on CorpusThomisticum (n.7):

"Quia si ad philosophiam primam attendamus, non possumus devenire in cognitionem divinarum et altissimarum causarum, nisi per ea quae ex virtute intellectus possibilis acquirimus. Si enim natura intellectus possibilis esset nobis ignota, non possemus scire ordinem substantiarum separatarum, sicut dicit Commentator super undecimo metaphysicae. Si vero attendatur quantum ad moralem..."

As you can see, there is a sentence absent in the Leonine text. The "apparatus criticus" makes no mention of the absence of this sentence, doesn't admit to its existence in any fashion, and hence gives no reason for why it wasn't included in the Leonine edition, or even a repudiation of its existence in other editions.

Now, on the level of sensus.

The first text seems to be saying that psychology is useful to first philosophy because it provides a sort of prerequisite knowledge, namely the knowledge of divine things and highest causes attained at the level of the soul, the "by those" (per ea) which (quae) we acquire from the power of the intellect. This seems intelligible enough as a general statement, that is, by examining human intellect, we build the groundwork for understanding the analogy (meant strictly) between our intellectual substances and higher ones. Or even, if "highest causes" is taken to include causes by predication, a case seems possible that by attaining a knowledge of our intellectual power, we must appreciate the distinctions between the various grades of potency and act, namely in corporeal change first, then in sensitive change (De Anima II.5) and finally in intellectual "motion."

However, it seems that the Marietti "additional sentence" gives us this reason explicitly. Namely, by knowing the nature of our possible intellect, we come to know the order of separate substances.

However, this position in itself runs into problems. First, St. Thomas is citing Averroes, who held the separate existence of the possible and agent intellects. Since St. Thomas disagrees with this position, it lends support to the Leonine text's excision of the sentence. However, this removal is, as mentioned, unexplained.

However, perhaps St. Thomas is quoting Averroes with reverence. That is, he is noting that, due to the analogy present between our intellects and higher ones, we must first know our own intellective powers, and upon this base an analogous structure to understand the order of separate substances.

More on this as it develops...