I'm working on an MA Dissertation on MacIntyre's thought. And I find that from his earlier rejection of Aristotelian metaphysical biology, he moves back into discussing human nature in a very Thomistic fashion. But my guide is quite disappointed with that, since he feels that we live in a post-metaphysical world, and that there is no thrill in reviving Aristotelianism if it is still metaphysical. I don't understand why that needs to be case. Is human nature necessarily a metaphysical concept?
There are many confusing definitions and ideas about what Metaphysics consists of; I like Aristotles, where he also calls it First Philosophy; he takes it to be an investigation into the first principles of nature.
To give an account, then of human nature, must then ground themselves into what these first principles are.
If a first principle is God, or Brahman or Allah; it seems that we must ground human nature there; and this is done, for in Christianity - we have the nature of the soul, in Hinduism - atman, in Islam - ruh.
In a framework where these are not accepted, say modern empiricism, then they are grounded naturally - in the mind, or subconscious: so Freud, Jung and Lacan.
So yes, by definition a full account must take metaphysics into account; a partial account will not, neccessarily.
Of course there are accounts of human nature which are not based on metaphysical concepts. You may take any contemporary textbook on psychology, e.g.,
- Zimbardo, Philip: Essentials of Psychology and Life
- Pervin, Lawrence; Cervone, Daniel; John, Oliver: Personality: Theory and Research"
and read about the methods and concepts used to investigate human nature, i.e. human affects, behaviour and cognition. Psychology has changed into an empirical science. It is not restricted by any traditional a-priori conception of human nature.
In addition, it is very risky to rely on Aristotle's basic concept nature. Sometimes he draws quite wrong conclusion from the concept, e.g.,
He then is by nature formed a slave who is qualified to become the chattel of another person [...]. Politics V, 1254b.
One of the challenges of exploring human nature physically is that, in our culture, the opposite of metaphysics is science. Functional physicalism actually makes the claim that human nature is explainable by the structure of a physical world. However, when we think of physics, we think of science. We think of the scientific method, testing hypotheses, and statistics.
This proves difficult when exploring the human condition, because typically one thinks of onesself as part of the human condition. One can test others scientifically, and make statements about how they operate. However, while exploring this, you have to ponder whether you are part of this "humanity" that you study. To resolve this question, you have to turn your study onto yourself.
However, there is a great challenge here. Science is extremely dependent on well controlled experiments. We take great effort to blind experimentees from knowing too much about the experiment, for fear of it ruining the results. However, it is impossible to do this on yourself. Thus science has a hard time grappling with the self, and physics thus has a hard time with it, simply because many of us define physics by science.
Relaxing the rule that physics is not always science, many other physicalist approaches become valid. Also, decreasing the formality of the language makes more approaches valid, if for no other reason than its increased ability to draw upon the human condition.