Plato and AristotleIn the dialogue Phaedo, Plato formulated his famous Theory of Forms as distinct and immaterial substances of which the objects and other phenomena that we perceive in the world are nothing more than mere shadows. Plato's doctrine was the prototype of all future manifestations of substance dualism in ontology. But Plato's doctrine of the Forms is not to be considered some sort of ancient and superseded metaphysical notion because it has precise implications for the philosophy of mind and the mind-body problem in particular.
Plato makes it clear, in the Phaedo, that the Forms are the universalia ante rem, i.e. they are universal concepts (or ideas) which make all of the phenomenal world intelligible. Consequently, in order for the intellect (the most important aspect of the mind in philosophy up until Descartes) to have access to any kind of knowledge with regard to any aspect of the universe, it must necessarily be a non-physical, immaterial entity (or property of some such entity) itself. So, it is clear on the basis of the texts that Plato was a very powerful precursor of Descartes and his subsequent more stringent formulation of the doctrine of substance dualism.
Aristotle strongly rejected Plato's notion of Forms as independently existing entities. In the Metaphysics, he already points to the central problems with this idea. On the one hand, if we say that the particulars of the phenomenal world participate or share in the Form, we seem to be destroying the Form's essential and indispensable unity. However, if we say that the particulars merely resemble, or are copies of, the Form, we seem to need an extra form to explain the connection between the members of the class consisting of the-particulars-and-the-form, and so on, leading to an infinite regress. This argument, originally formulated by Plato himself in the Parmenides, was later given the name of the "third man argument" by Aristotle.
For these reasons, Aristotle revised the theory of forms so as to eliminate the idea of their independent existence from concrete, particular entities. The form of something, for Aristotle, is the nature or essence (ousia, in Greek) of that thing. To say that Socrates and Callias are both men is not to say that there is some transcendent entity "man" to which both Socrates and Callias belong. The form is indeed substance but it is not substance over and above the substance of the concrete entities which it characterizes. Aristotle rejects both universalia in rebus as well as universalia ante rem.[ Some philosophers and thinkers have taken this to be a form of materialism and there may be something to their arguments. However, what is important from the perspective of philosophy of mind is that Aristotle does not believe that intellect can be conceived of as something material. He argues as follows: if the intellect were material then it could not receive all of the forms. If the intellect were a specific material organ (or part of one) then it would be restricted to receiving only certain kinds of information, as the eye is restricted to receiving visual data and the ear is restricted to receiving auditory data. Since the intellect is capable of receiving and reflecting on all forms of data, then it must not be a physical organ and, hence, it must be immaterial