An Avicennan-style enthymematic argument for the perspicacious (ahl-al-fatana)*:
Any physical entity, taken just by itself, is indeterminate or indifferent to any possible range of ma’āni. Therefore, the mind is immaterial.
Nothing material, whether it is a substance or some modification thereof, such as a process, or an artifact, like a machine, considered purely in terms of its corporeality, determines or fixes one meaning as opposed to any other. Take, for example, some pattern of neuronal activity in Zayd’s brain, however complex you imagine it may be. Considered only as such, neither the activity as a whole nor the elements which make it up are determinately about this meaning (e.g., horseness) nor that (e.g., bovinity). And hence from simply examining such physical states, what they mean, which of the two Zayd is thinking of, can in no way be determined. In other words, such states are equally indifferent and hence compatible with both – nay, in terms of just their physical constituents, they are consistent with an infinite amount of other meanings. But it is manifest from experience that Zayd’s thoughts have determinate conceptual content; that is, they uniquely instantiate e.g., horseness and nothing else. Therefore, etc.
Neither the phenomenal content (qualia), nor the intentionality, of Zayd’s perceptual experiences is the middle term in the present argument. Both things are properties of non-human animals; and although both are teleologically directed, they are still indeterminate in the relevant sense. For exactly what Zayd hears cannot be specified uniquely on the basis of only the physical elements involved in the fact that he hears. As such, they are not, on our view, sufficient to distinguish the immaterial from the material.
You will perhaps object: granted that no single pattern of Zayd’s neural activity fixes meaning, but why can’t the interconnected set or system of such neuronal activities (in his brain) do it? Its falsity requires proof.
Response: If a single pattern, qua material, is indeterminate, then the set of neural patters, precisely qua material, are indeterminate. Not every inference from the parts to the whole commits the well-known fallacy. Moreover, what is denied is not that the set or system can specify meaning in the sense of narrowing down meaning. Arguendo, perhaps it can. Rather, what is denied is that it can fix a uniquely single meaning from among the (infinite) range of possible ma’āni.
If you insist: Perhaps, then, neural states, whether individually or as a set, represent determinate meanings by virtue of the causal relation they bear to objects or events in the external world.
Response: That move commits the petittio fallacy; for the relation between the pattern of neural activity and the determinate meaning it is assumed to represent or fix is just as indeterminate as the relation between the objects or events in the external world and the neuronal activity they are assumed to cause. From the physical facts alone, nothing about, say, the hay and the neuronal pattern it sets off in the horse indicates that the latter uniquely represents hay and not, say, fake hay or fresh hay or old hay, all of which are different meanings and all of which the horse is indifferent to and so will equally devour.
Nothing in the physical facts alone of any given causal chain – in this case the objects in the world and neurons they purportedly activate (O+N) – dictates that such a chain determinately represents some one meaning e.g., horseness. The case here is no different from some artificial object e.g., the sign “@” and what it represents. In itself, perhaps it refers to just the letter ‘a’ or to the vowel ‘a’ or it can equally refer to some obscure mathematical concept, or something else, and so on. Any definite meaning it will have will be determined, not by its materiality, but by minds which make use of it. Similarly, nothing about (O+N) necessarily instantiates, or refers to, any unique meaning – in this case, the concept which Zayd thinks about when he engages in an act of thought. (O+N), then, is consistent with other meanings, whereas Zayd’s thought is of a wholly determinate form, i.e., horseness and nothing else.
If you object: But on your view, the causal powers are directed towards determinate and specific effects. Hence, if what you said above is granted, you have contradicted yourself. Otherwise, why can’t, on your view, such causal chains be determinately directed towards triggering certain determinate patterns of neuronal activity, which then in turn represent determinate meanings?
Response: There’s an equivocation on the term ‘determinacy’ on your part, and so your conclusion does not go through. That it is a form of determinacy, I grant; but that it is the kind our thoughts have, I deny. Hence, what the argument shows is that whatever directedness various physical things and their causal powers have, they cannot, as purely material, have the relevant determinacy that our concepts and judgments have. In the external world, substances and their causal powers only approximate the latter; that is, they only instantiate it indeterminately. For no particular horse perfectly instantiates horseness and the activities which follow upon that form; no particular circle perfectly instantiates the form circularity and the accidents which follow upon it in the external world. And so on with the rest. Therefore, that “A is an efficient cause of B and B is the final cause of A” is only a linguistic *representation* – by our minds to be sure – of an intelligible or abstract form our intellect grasps. But our intellect’s instantiation or grasp of such forms is perfect and uniquely determinate.
As for the proof that material substances, despite their directed activities, instantiate their causal structures only indeterminately, it is as follows:
What instantiates a form imperfectly ipso facto instantiates the causal powers of that form imperfectly.
Material substances instantiate forms imperfectly.
Therefore, they instantiate the causal powers of those forms imperfectly
What is instantiated imperfectly is indeterminate (i.e., in the relevant sense).
Therefore, such causal powers are indeterminate (i.e., in the relevant sense).
Wa Allahu A’lam.
*For the full details of the argument, see E. Feser, “Kripke, Ross, and the Immaterial Aspects of Thought”, American Catholic Philosophical Quarterly, 87 (2013): 1-32; and J. Ross, “Immaterial Aspects of Thought,” Journal of Philosophy 89 (1992): 136–50.