The Soul is the Starting-Point for Living

"The substance (οὐσία) is the cause of existing, and here, in the case of living things, to exist is to live, and the soul (ψυχή) in them is the cause (αἰτία) and starting-point (ἀρχὴ)" (On the Soul II.4.415b12-15).

"Substances (οὐσίαι) most of all are thought to be bodies, especially natural bodies, for they are the starting-points for other bodies. Of the natural bodies, some have life and some do not. Life we say is self-nutrition and growth and decay. Thus every natural body having life is a substance as a composite (συνθέτη). But since it is a body of a definite kind, viz., having life, the body cannot be soul, for the body is not something predicated of a subject, but rather is itself to be regarded as a subject, i.e., as matter (ὕλη). So the soul must be substance as the form (ψυχὴν οὐσίαν εἶναι ὡς εἶδος) of a natural body, which potentially has life. And substance is actuality (ἐντελέχεια). The soul, then, is the actuality of a body. But actuality is of two kinds, corresponding to knowledge (ἐπιστήμη) and contemplation (τὸ θεωρεῖν). The soul is actuality like knowledge. Sleeping and waking presuppose the existence of soul, and waking corresponds to contemplation, sleeping to knowledge since it comes first. That is why the soul is the first actuality (ἐντελέχεια ἡ πρώτη) of a natural body having life potentially in it" (On the Soul II.1.412a).

"We speak of the soul as being pained or pleased, being bold or fearful, being angry, perceiving, thinking. All these things are regarded as modes of movement, and hence it might be inferred that the soul is moved. This, however, does not follow. We may admit that being pained or pleased, or thinking, are movements, and that the soul is the cause of these movements. For example, we may regard anger or fear as such and such movements of the heart.... Yet to say that the soul is angry is like saying that the soul weaves or builds houses. Doubtless it is better to avoid saying that the soul pities or learns or thinks, and to say instead that the man does this with his soul (ἄνθρωπον τῇ ψυχῇ)" (On the Soul I.4.408b1-15).
All natural bodies have natures. The nature is the "starting-point of change and staying unchanged." It is the organization of the material so that there exists a body of the kind. These natures are forms, and these forms are in matter and separate only in account.

In the case of living natural bodies, Aristotle identifies the form is a "soul" (ψυχή).

The Soul is a First Actuality

Aristotle says that the soul is a "first actuality" (ἐντελέχεια ἡ πρώτη).

The word "actuality" (ἐντελέχεια) is one Aristotle seems to have invented. It is a compound of three words: "in" (ἐν), "end" (τέλοϛ), and "hold" (ἔχειν).

By describing the soul as a "actuality," Aristotle means to say that the soul is the is the organization of material so that there is an object who throughout its existence "holds" some end. Living is this end. The soul is the starting-point for living. Living is the end for the sake of which the material in the body has its organization. In this way, the soul "actualizes" the material.

By describing the soul as a "first actuality," Aristotle qualifies the way the soul is an "actuality." The qualification is "first." Aristotle does not explain very clearly what this qualification is, but the suggestion is that the end the soul "actualizes" is an end with qualification.

So Aristotle's idea, in calling the soul a "first actuality," is that the soul is the organization of the matter so that the matter constitutes a living natural body. In this way, the soul has an end. It is for something. It is for living. In the case of a human soul, the end is living a human life. This end is an end with qualification. The end without qualification is a different existence.

Aristotle Corrects Two Mistakes

By conceiving of human beings as natural living bodies, and by conceiving of natural bodies as forms in matter, Aristotle tries to correct what he understands as Plato's mistakes.

Plato thought that the soul is an object that can exist independently of the body. Aristotle thinks this is a mistake. The soul is not separate without qualification. As the form of a natural body, the soul is in matter. It is the organization of the material so that there exist a living body. This organization is not separate without qualification. It is separate only in account.

Plato, following Socrates, thought the soul can change and that it can be in a better or worse state. Aristotle thinks this is a mistake. Because the the soul is the organization of the material into a living natural body, the soul is not the kind of thing that can change. A human being can change. In this, however, it is the human being, not the soul, that changes. The soul makes the change possible. It is the starting-point for change, but the soul itself does not change.

Perseus Digital Library:

Aristotle, Metaphysics.
Henry George Liddell, Robert Scott, A Greek-English Lexicon:
ἐντελέχεια, entelecheia, noun, "full, complete reality,"
ὕλη, hylē, noun, "matter,"

Arizona State University Library: Loeb Classical Library
Aristotle, On the Soul

"The forms of sensible substances involve potentiality in two ways, and hence are not pure actualities, though it is the essence of a form to be an actuality. They need matter to be realized in, and thus are the forms of objects subject to change. But, what is more, when we turn to the paradigms of sensible substances, living beings, it turns out that their forms themselves essentially contain an element of potentiality. When Aristotle in De Anima [= On the Soul] II.1 defines the soul as the 'first actuality' of a certain kind of body, this very language reflects the fact that the soul in a way is constituted by the various abilities to exercise the life- functions characteristic of the kind of living being in question, but that not all these life-functions are exercised all the time. What is more, some of the abilities that characterize the soul, like virtue and knowledge, are only acquired. Thus, the forms of sensible substances are not pure actualities; they in part are constituted by unrealized possibilities and in that sense are not fully real. The form that is the unmoved mover, on the other hand, is pure actuality. It neither needs matter to be realized nor does it involve any abilities that might or might not be realized or exercised. The unmoved mover is just eternally thinking the same thought." (Michael Frede, "The Unity of General and Special Metaphysics: Aristotle's Conception of Metaphysics," 89-90.)

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