Aristotle (De Anima, I. 1) makes in the first place the general remark that it appears as if the soul must, on the one hand, be regarded in its freedom as independent and as separable from the body, since in thinking it is independent; and, on the other hand, since in the emotions it appears to be united with the body and not separate, it must also be looked on as being inseparable from it; for the emotions show themselves as materialized Notions (λόγοι ἔνυλοι), as material modes of what is spiritual. With this a twofold method of considering the soul, also known to Aristotle, comes into play, namely the purely rational or logical view, on the one hand, and, on the other hand, the physical or physiological; these we still see practised side by side. According to the one view, anger, for instance, is looked on as an eager desire for retaliation or the like; according to the other view it is the surging upward of the heart-blood and the warm element in man. The former is the rational, the latter the material view of anger; just as one man may define a house as a shelter against wind, rain, and other destructive agencies, while another defines it as consisting of wood and stone; that is to say, the former gives the determination and the form, or the purpose of the thing, while the latter specifies the material it is made of, and its necessary conditions.
Aristotle characterizes the nature of the soul more closely (De Anima, II. 1) by referring to the three moments of existence: “First there is matter (ὕλη), which is in itself no individual thing; secondly, the form and the universal (μορφὴ καὶ εἶδος), which give a thing individuality; thirdly, the result produced by both, in which matter is potentiality and form is energy (ἐντελέχεια);” matter thus does not exist as matter, but only implicitly. “The soul is substance, as being the form of the physical organic body which is possessed potentially of life; but its substance is energy (ἐντελέχεια), the energy of a body such as has been described” (endowed with life). “This energy appears in twofold form: either as knowledge (ἐπιστήμη) or as active observation (τὸ θεωρεῖν). But it is evident that here it is to be regarded as the former of these. For the soul is present with us both when we sleep and when we wake; waking corresponds with active observation, and sleep with possession and passivity. But knowledge is in origination prior to all else. The soul is thus the first energy of a physical but organic body.” It is in respect of this that Aristotle gives to the soul the definition of being the entelechy (_supra_, pp. 143, 144).