There are very many arguments that tell us that the substance of the mind cannot be material. Mental function cannot be derived from physical causality and still depend on rationality and teleology, because physical processes are formulated not to depend on reasons or purposes. Minds must therefore be non-physical, if they are indeed to depend on rationality and purpose. But 'non-material' and 'non-physical' are negative characterizations, and do not tell us what minds actually are!
Descartes postulated that minds were 'essentially rational': that rationality was the essence of mind. Just as spatial extension was the essence of matter. However, this still does not tell us the substance of mind, or what the causal powers of minds are specifically. There obviously are causal connections between mind and body, but Descartes' characterization of 'minds as rationality' does not tell us what these are.
But now we can use the basis of ideas described in my recent posts "Mental dispositions and desires" and "Are propensities the substance of things". In the first-mentioned post, mental activity was taken to consist of the operation of persistent dispositions or desires or propensities. In the second-mentioned post, persistent propensities were taken to be the substance of which a thing would be formed.
Combining these, we conclude:
mental activity is the operation of mental substances such as the persistent mental dispositions.
This is in exact analogy to the way physical things are formed of physical propensities (like gravity, electromagnetism, etc), which makes them to be physical substances. Minds are made of persistent mental dispositions. Those dispositions are the underlying motivations, loves and desires which make the person behave the way they tend to do. This claim may be summarized as:
The mental substance is Love
More precisely: mental substances are the underlying persistent loves which cause mental activities. By 'persistent', I mean at least lasting in time from one event to the next event after some discrete non-zero time interval. Maybe persistent loves last longer, such as for a whole lifetime: that remains to be investigated.
In this account, we may take consciousness to be a feature of the operation of loves and dispositions, and not that minds is made out of consciousness as if it were a substance. I argue that consciousness cannot itself be substance, on the basis of the Aristotelean ‘metaphysical grammar’ we are following. Only something dispositional like love or power can be a substance.
There is much talk in 'consciousness studies' about how the appearances of things, qualia, are missing from current physical theories. Consciousness is then claimed to necessarily exist in addition to the physical. This last step, I suggest, is a category mistake. 'Consciousness' is an operation, not a substance. It does not 'exist' in the way substances exist, as William James long ago realized.
Love has causal powers, and can make changes in physical things (as we all know!). Love always motivates from some purpose. In humans, we think it ought to be possible to link love with rationality and foresight, in such way that purposes and reasons come to have causal powers in both the mental and then in the physical realms. But exactly how these things occur has yet to investigated.
Note finally that, though I have following an Aristotelean logic, I have arrived a position almost of the opposite of that formulated by Thomas Aquinas. Above, I take minds to be essentially some mental substances, in some forms or structure. Aquinas, however, takes minds to be the forms of things that is conjoined with potency: material potency in the case of embodied creatures. So, is mind the substance, or is it the form?
The resolution of this conflict will be explained later in more detail, and will depend on realizing that Aquinas, by the term 'form', refers to much more than shape or structure. In fact, he takes 'form' to refer to the general/deep causal principles that makes a thing what it is: its 'substantial form'. This, in my opinion, stretches the meaning of 'form' much too far, but is what he must do without the concept of substance I have been recently describing.