Aristotle famously describes man as a rational animal. We may not appreciate the depths of Aristotle's view if we interpret him within the modern evolutionary categories that are our default intellectual equipment. We probably imagine, sometime in the past, an animal like any other animal that, through evolutionary circumstance, happened to develop a particularly clever brain. Our picture is that of a layer of rationality imposed on an irrational animal nature underneath. This isn't Aristotle's view.
Consider the start of Book II of the Nicomachean Ethics. Here Aristotle discusses the relationship of the virtues to nature. The virtues cannot be contrary to nature, or it would be impossible to achieve them. Nor do they come to us by nature, for then no effort would be required to obtain them. "So virtues arise in us neither by nature nor contrary to nature, but nature gives us the capacity to acquire them, and completion comes through habituation." (From the Cambridge Texts in the History of Philosophy version of the Nichomachean Ethics).
Not all acts conduce to virtuous development, however. Which ones do? That is a question for reason to determine. Given that habituation to virtues perfects human nature, we are left with the following conclusion concerning man: His nature is constituted such that its development and completion is possible only through a course of action prescribed by reason. This is a remarkable statement, for it means that what we think of as the "irrational", animal part of man's nature is ordered to reason; rationality, for Aristotle, is not limited to the roof of man's nature but penetrates all the way to the basement. Reason is to man's nature something like the way the sun is to a tree's nature; the tree's leaves may be the immediate interface to the sun's energy, but the entire nature of the tree is ordered to the capture and exploitation of solar energy. Similarly, "the brain" may be the immediate organ of reason, but man's entire nature is ordered to the development of, and subjection to, reason.
The analogy is far from perfect. For one thing, the sun is external to the plant's nature, but reason is internal to ours, and indeed constitutive of it. This is why we are free in a way that plants and animals are not. The plant's nature is immediately ordered to an external being; our nature is only indirectly ordered to it, as the truth of our end discovered by reason. Our nature is immediately ordered to reason; rather than blindly following the sun, we follow the truth as we come to know it, the truth about ourselves, the universe, and God. This is what it means to be a rational animal.