a theory of mind
The most common conception of mind that we have today is that we have some sort of free will, some sort of faculty that is most closely identified with the executive decision making center of the self, which we might call the ego. We have many drives, physiological, emotional, etc. which the ego alone has the ability to impartially judge. Sure, the ego is influenced by all of these drives, but we believe that it is ultimately free by virtue of its nature as the ego. It might have been a hard decision to make, but ultimately, we could have always chosen whatever we wanted.
But apart from our cultural background which, reinforced by experience, seems to lead us to this conclusion, it is not immediately apparent that this model of the mind is the best one. Even given that the ego makes choices in some capacity, it seems unreasonable to suggest that it has complete freedom when the choices that appear to it are already bounded by what it is aware of.
Perhaps equally unsatisfying is some sort of physical determinism, where given the "laws" of physics the trajectory of every particle has already been determined in an endless chain of cause and effect, which naturally involves our minds (brains) and bodies.
Morality, too, is relative for the modern. It is a matter of preference. We enter into some sort of social agreement about what is considered acceptable but ultimately we do not believe that certain things must be universally wrong or evil. We may feel very strongly one way or another, but it is just as reasonable to suppose that a person brought up in a different kind of culture may have different moral feelings on the same matter.
Nietzsche proposes a model of the mind which consists of competing drives. There is no "free" will, only weak and strong wills. When torn between two choices, the two wills engage in battle until the stronger one wins, and we rationalize to ourselves after the fact that we chose that option. In this sense the human being is just an animal that discovered knowledge. Nietzsche also recognizes that with the death of God there is no objective basis for good and evil. However, he believes that it is the philosopher's role to legislate new values.
Kant, although I barely understand his metaphysics, seems to suggest that while we have instincts and are influenced by them, we have some sort of rational faculty which is able to exercise its freedom only by acting morally. And the categorical imperative, although dubious from a modern viewpoint, does prescribe a form of universal morality.
Rousseau proposed similarly that there is a universal "law of nature" which our conscience is naturally aligned with. For both Rousseau and Kant human beings are capable of acting rationally. For Nietzsche, this is unclear.
But clearly we do experience "rational" decision-making, or at least it seems that way to us. Everyone has the experience of making a decision guided by reasons that they themselves came up with or accepted as being reasonable. Nietzsche would not deny this, but he would suggest that these reasons are made after the fact in order to justify the instincts we are ultimately acting to satisfy.
I remember distinctly the moment I first began to break free of my instincts and exercise what seemed to be a free, rational will. When I was a kid, probably 4 or 5 years old, it was natural to me to get upset and express that should I not get what I wanted. But there was one day where I realized there was no point in throwing a fit. Getting upset probably wouldn't change my parents' decision, and the things I was getting upset over were trivial. So I "decided" to stop getting upset.
This is definitely a real experience that I had, and that many people share. So does this mean that we do have a free and rational will? It's difficult to say. That's just one way to interpret this experience. And here is where morality comes in.
When I "made" that choice to stop throwing tantrums, it was not arbitrary. There was some sort of moral basis for it, probably taught by my parents. It's unlikely they were cheering me on and encouraging me to throw fits whenever I did. More likely, they were disapproving. It could be argued that this introduced to me a moral judgment, which eventually became strong enough to assert itself. There's no need for a free will in this scenario. One day, the moral impulse just became stronger than the impulse to act out the other instinctual impulses.
This is why Nietzsche calls good and evil the most powerful force. It has the power to transform the nature of human beings.
On the other hand, we could keep the free will and keep morality and say that I became aware of moral values and decided to act in accordance with them.