Many authors, especially in introductory books, seem to characterize Aristotle's forms as some kind of structure or organizational feature of matter, which seems to be compatible with a reductionist view of nature.

Others characterize it as a holistic 'something', more than what can explained by structure, but still wholly dependent on the matter in which the form 'resides'. I don't know which view this is correct. Also, some authors claim that it cannot be objectively reconstructed how Aristotle understood forms.

Now, to complicate this even more, Aquinas and other medieval philosophers, held that the soul is the form of the body.

Some of the (arguably) most difficult unsolved problems in philosophy are consciousness, intentionality and rationality. How can Aquinas' theory explain those?

Doesn't Aquinas want our thinking to have observable, objective, material effects? So does the soul causally act somehow? But in that case, how can we still speak of the soul as a form?

Also, Aquinas also believed in the immortality of the soul, i.e. the soul as form can continue to exist even after fundamental change in the corresponding matter: biological death – the destruction of the body.

How can this be possible? What is the most natural understanding of Aristotelian forms which is compatible with the claim that the soul is the form of the body and can continue to exist (where?) all by itself after the destruction of the body?

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    For reference, Aristotle also believed in the immortality of human soul (it's not singular to Aquinas). You can find the argument in NE Book X among other places.
    – virmaior
    Commented Dec 10, 2016 at 12:52
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    Also, I think you're dealing with a lot of the right issues here, but I'm not quite grasping what the incompatibility is between soul as causal and soul as form (which for Aquinas / Aristotle) means the organizing principle of a thing. Can you unpack what makes them seem incompatible?
    – virmaior
    Commented Dec 10, 2016 at 12:53
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    Soul is the form of the body according to Aristotle; see Aristotle's Psychology : "In De Anima, A claims, using vocabulary derived from his physical and metaphysical theories, that the soul is a “first actuality of a natural organic body” (De An ii 1,412b5–6), that it is a “substance as form of a natural body which has life in potentiality” (ii 1,412a20–1) and, similarly, that it “is a first actuality of a natural body which has life in potentiality” (ii 1,412a27–8), all claims which apply to plants, animals and humans alike." Commented Dec 10, 2016 at 14:38
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    Soul is not mind (in modern sense); the soul is the "form" of the living being because it is the cause (form=essence=formal cause) that accounts for (explains why) this matter is the matter of a human being, as opposed to some other kind of thing. Commented Dec 10, 2016 at 14:42
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    See also Ancient Theories of the Soul : Aristotle : "Given that the soul is, according to Aristotle's theory, a system of abilities possessed and manifested by animate bodies of suitable structure, it is clear that the soul is, according to Aristotle, not itself a body or a corporeal thing. [...] Aristotle seems to be committed to the view that, contrary to the Platonic position, even human souls are not capable of existence and (perhaps as importantly) activity apart from the body (cf. De An I 1, 403a3-25, esp. 5-16)." Commented Dec 10, 2016 at 14:46

2 Answers 2


What seems to be omitted is that Aristotle was a realist about forms. He does call them secondary substances whose existence depends on the existence of primary substances (particulars, e.g. material things), but he does say that both exist in re, see Aristotle's Theory of Abstraction by Bäck, Ch.2. Furthermore, the "structure" is not a mere epiphenomenon of material interactions, as understood today (Averroes called something like that a "material" form, as opposed to "substantial" one), matter itself is only potential, it is form that brings potential into actual in what Aristotle calls entelechy (roughly, embodiment, actualization).

Dependent existence means, and this is a difference with Plato, that forms do not float separately from their particulars, which are the unities of matter and form, however, transplanted into a receptive medium, they could be separated. Normally, the receptive medium is human intellect (Greek nous) that abstracts forms from their material instantiations. As for the soul specifically, Cohoe investigated the issue in detail in his recent thesis The Human Intellect: Aristotle's Conception of Nous in his De Anima, who concludes the following:

I consider whether the separability of intellectual activity from the body would be compatible with Aristotle’s overall views on the soul and its relation to the body, particularly his claim that the soul is the form of the body. I present two alternative interpretations of Aristotle’s conception of the soul: (1) the soul is constituted by a unified and interrelated set of powers; (2) the soul is ontologically prior to its powers and is not constituted by them. I conclude that both interpretations are compatible with the relevant texts... If the intellectual power and its activities can exist separately, then when, after the destruction of the human body, they do exist separately, the human being also continues to exist. I argue that this view is preferable for textual and philosophical reasons to a position according to which the primary subject of understanding is the intellect or the soul or one according to which understanding switches primary subjects after death.

"Where" the soul might exist is a curious question, but Aristotle did have Divine Nous. Even on the first reading there might perhaps be a way to make Aristotle into something like a functionalist (with a good deal of stretching, in my opinion). Goldberg tries it in Is Aristotle's Philosophy of Mind Functionalist?, but even he calls him an "antireductionist physicalist", as "the only ontology that Aristotle countenances, at least in the sublunary realm, is one of ordinary matter. Nonetheless souls, like syllables, are not reducible just to that matter; they are matter in particular forms." But even Davidson's nonreductionist anomalous monism (no mental substance but irreducible mental properties) does not make forms as much in re as Aristotle does.

Aristotle's obscurity on the soul led to diverging interpretations during the middle ages, first by Islamic and then by Christian philosophers, to whom the theological concerns made the issue paramount. The two most inluential ones were fashioned by Avicenna, who augmented Aristotle with neo-Platonic elements borrowed from Plotinus, and Averroes, whose conception influenced Aquinas. Although Aquinas, unlike Aristotle or Duns Scotus, was not a realist about universals he was most certainly a realist about souls as substantial forms. But he added a special twist that anticipated Kant and distingushed him from all other scholastics, see Aquinas: Body and Soul on SEP.

Aquinas did not consider existence to be a property, part of essence, but characterized it rather as an act, esse. While a soul without a body is incomplete in essence, as Aristotle held, it can still subsist through its immaterial esse, and the body itself receives its existence through the same esse when it is insouled. This allowed Aquinas to maintain the tight Aristotelian link between body and soul in his largely empiricist theory of cognition, but at the same time allow for survival of the soul upon destruction of the body through esse, as the Scripture demanded. This survival is however deficient, soul is only "subsistent" through incorporeal activities like thinking and willing (interestingly, animal souls are not), but it is not a "substance complete in essence" (although God might perhaps convert it into such an "angelic" substance after the resurrection). The last two chapters of Kenny's book Aquinas on Mind give a detailed account of Aquinas's view of the nature of the soul and its relation to the body.

As for observable effects, there is no problem with soul affecting matter, in Aquinas's metaphysics the higher can always affect the lower, but not vice versa. So soul can act on inert matter but not vice versa, see Aquinas and the Content Fallacy. Aquinas faced the opposite problem for his empiricism, however: how could matter (the lower) affect the intellect (the higher) in perception and cognition, even going through senses. This he solved by appropriating an entity barely mentioned by Aristotle and made into a part of the Divine Nous by Avicenna, and making it a part of human soul, see What is the agent intellect according to Avicenna and Aquinas?:

"It is necessary to postulate a power, belonging to the intellect, to create actually thinkable objects by abstracting ideas from their material conditions. That is why we need to postulate an agent intellect."

Instead of "receiving" material forms through senses, which would require matter acting on intellect, the agent intellect acts the other way, by actively processing what is received through the senses (stripping forms of their material conditions). This was a novel idea, also anticipating Kant with his active cognitive subject. Let me finally mention that Kant modeled his sensation/concept unity in perception on the Aristotelian matter/form unity in re.

  • 1.) "Realism about forms" doesn't say much, if it is not clear what forms are. 2.) Now how does the soul survive after death? Do I understand you correctly, that you claim Aristotle meant that human souls are transferred into another medium after death? Divine Nous? What is peculiar about human souls that makes this possible (contrary to animal souls)? 3.) Aquinas was not a realist about universals? There is no consensus about this, it seems.
    – viuser
    Commented Dec 12, 2016 at 21:08
  • 4.) It still seems puzzling how forms can act out of themselves but be so dependent on matter on the other hand. 5.) The interaction between soul as form and the body (brain) seems as problematic as in Cartesian dualism.
    – viuser
    Commented Dec 12, 2016 at 21:10
  • @wolf-revo-cats We can not say more than what survived in Aristotle's writings, it does not appear that he addressed the issue (or cared), persistence in Nous was Avicenna's suggestion, Averroes held individual souls perishable, only mankind-soul persists. Aquinas is typically described as a (proto)conceptualist, but I suppose he was sufficiently close to Aristotle for there to be a controversy. Human souls are special (perhaps implicitly even for Aristotle) for Aquinas due to esse. He asserts that God's esse is also his essence, "He is that He is". And humans were created in God's image.
    – Conifold
    Commented Dec 12, 2016 at 22:51
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    @wolf-revo-cats I think Pasnau's surmise in From and Matter is as definitive as it gets:"In Aristotle, these two aspects of form – proto-scientific and metaphysical – exist side by side, so that sometimes forms are conceived of on the model of souls, where souls are thought to have certain causal powers, whereas at other times forms are conceived of as abstract, functional principles, offering explanations at a level that is quite independent of whatever causal, physical story might be told about the natural world".
    – Conifold
    Commented Feb 28, 2017 at 21:00
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    Kenny says the same thing and goes further:"But besides the abstract notion of form, there is a notion of form as an agent. In these passages it is clear that Aquinas thinks of the human soul as being causally responsible for the various activities which make up a human life. And here the causality is efficient causality... It is this kind of relationship that is suggested when we are told that the soul is the principle of life... The two notions of form seem to be different from each other and impossible to combine, without confusion, into a single notion".
    – Conifold
    Commented Feb 28, 2017 at 21:02

Soul, in at least one sense is for Aristotle a principle of change.

To start at the beginning: Lets takes some entity, now it can be made to change into two mutually exclusive ways; either it can be made to change by the action of another, or by itself - this is a straight-forward, logical distinction.

It's the second sense that is used in soul; it changes itself - it grows, it alters, it moves; its a self-mover.

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