Human beings have long been described as the rational animal. It is an evolutionary possibility that the description is accurate.
None of the beliefs we acquire throughout our lives is a biological adaptation. What is an adaptation is the ability to acquire beliefs. In order for the latter to be an adaptation, the kind of beliefs that we are biologically programmed to acquire must be those that are advantageous (in that they enable us to increase our genetic legacy by making us more viable or more fertile). But it is frequently impossible to distinguish advantageous from disadvantageous beliefs at the time of acquisition. Their effects on our genetic legacy become apparent only when we have occasion to act on them in entirely different circumstances and at a later time, and they do not always become apparent even then. Therefore, we need a property to act as a proxy for advantageousness. The only plausible candidate for such a proxy is truth qua correspondence. Advantageousness is the ultimate goal but truth is the proximate goal. As truth-seekers, we rely on evidence for and against propositions, and on the logical relations between them. This reliance on evidence and logic is what it is to be rational.
Our naturally selected capacity for rationality is like our immune system in many respects. That we have an immune system does not entail that we never become ill, that we never encounter illnesses to which we have no immunity, that we cannot suffer accidents, or that there are no auto-immune disorders. Likewise, our reasoning may be imperfect in a variety of ways. There is variation in immune systems. Likewise, there is variation in our capacity to reason. The fact that we have an immune system does not entail that we cannot do more to maintain our health. The fact that we have a natural capacity to reason does not entail that we cannot improve it in various ways.
It does not follow that all beliefs have to be acquired beliefs, that is, products of our naturally selected capacity to acquire beliefs. It is possible that some beliefs are themselves adaptations. If they are themselves adaptations, they are advantageous, full stop, and it is unnecessary to rely on a proxy. If they are advantageous, full stop, it does not matter if they are false. There will still be selection for them once they have arisen.
The theory to be presented here, evolutionary intuitionism, is based on just such a naturally selected belief. We have not acquired it in response to evidence that it is true. The naturally selected belief can be expressed as “I am of value, full stop.” It is this belief that is, ultimately, the basis of morality.
A human being is both a rational animal and a moral animal. A human being is not a moral animal because he is a rational animal. Nevertheless, the way to become a morally better human being is by becoming more rational, as we shall see.