Neurological research ha been done in which people's brains are observed while they undergo changes in their subjective internal states. By this I mean that they are told to try to bring about certain thoughts, ideas, emotions, intentions &c. The researchers can then correlate these internal states with the brain states that they observe. This tells them more about the structure of the brain and its relationship to our thoughts, emotions &c. Certainly a noble and interesting area of research, one that might be useful in the diagnosing and treating of certain physical and chemical disorders that might lead to mental and emotional problems.
But scientific materialist do more than recognize the measurable correlation between these subjective internal states and their correlating brain states. They argue that the cause of these subjective internal states are the correlating brain states. The argument goes as follows:
If a person is subject to some internal state, then their brain has a brain state that corresponds to that internal state.
A person's brain has a brain state that corresponds to some internal state.
Therefore the person is subject to that internal state.
This is an invalid argument. While the conditional statement is simply a restatement of the research data, the rest of the argument is an example of the fallacy of affirming the consequent.
It may be true that particular brain states are necessary for certain subjective internal states. But this is certainly not a problem for an Aristotelian or a Thomist, whose philosophical psychology and philosophical anthropology requires the activity of the internal senses for any kind of thought: "The Philosopher says (Metaph. i, 1; Poster. ii, 15) that the principle of knowledge is in the senses" (Summa Theologiae I, q. 84, a. 6 sed contra). It does not follow from the research, however, that particular brains states are a sufficient reason for certain subjective internal states. If scientific materialists are correct, at least insofar as human nature is concerned, they will have to do more than point to these kinds of studies to prove it.