De causis curatricibus Theses Progymnasticae

not every soul is Nature, but one or even several parts of it.

XLIX.
Yet, either Aristotle was speaking only with the common consensus of other philosophers and wished to save the diverse phenomena of the generation and perfection of man, and thus was less exquisite or proper in his speech; or (which is perhaps more likely according to the principal interpreters) he understood by the name "parts" the powers, distinct at least by reason and definition, but not really or substantially, or according to the subject, which is what Plato wanted, against whom he also argues. Elsewhere, he teaches differently, as is clearly evident in book 1, chapter 5 of On the Soul, and 3 On the Soul, chapter 9.

L.
Even if diverse works appear—some of which seem proper and private to the intellectual soul, some to the sensitive, and some to the plant-like—it is probable to him and to others who inspect the matter more accurately that there is one Soul of Man, not several, which is the efficient cause of all these. For exact philosophy judges things correctly not according to appearance, but according to essence and truth itself.

LI.
Indeed, the One who made the Soul did not assemble it from several substances, parts, or species; but as He made it noble and perfect, so He also fashioned it very simply. He instructed it with several powers—fixed and the same—and imbued and informed it with precepts, partly more general or common, and partly more contracted or specific.

LII.
With the more general ones (namely, the Sensitive and Vegetative), it moves the body, and that too with various motions: especially, however, the Vegetative maintains its union with the body and is the author of its every health. With the more contracted or specific ones, it moves almost only itself, and does not communicate these to the body as nobly as the Sensitive and Vegetative do—that is, it understands and reasons. Thus, the human Soul becomes the most perfect Form of all natural things, and not only contains the perfections of the inferior (that is, plants and animals, while it constitutes the body with matter, is the principle of motion and nutrition, and is also the cause of sensing) but also participates in a superior perfection, that is, it is the principle of understanding; and thus, although it is one in substance, it nevertheless appears multiplex in virtue and power.

LIII.
Furthermore, since it is necessary for something to exist between the thing moved and the mover, and since therefore the Soul cannot produce bodily motions immediately (since an ethereal and spiritual substance cannot move a corporal and earthly one without a suitable medium, especially one that is a participant in both natures), it therefore accepted certain instruments and organs for those motions of a more general power—and these not simple, but (since the method of governing and preserving these motions of the body requires nothing if not variety) also themselves various and diverse.