For Nagel, the existence of rational animals is not a brute fact or fluke or cosmic accident. Nagel's somewhat sketchy argument (see p. 86) is along these lines:
1. There are organisms capable of reason.
2. The possibility of such beings must have been there from the beginning.
3. This possibility, however, must be grounded in and explained by the nature of the cosmos.
4. What's more, the nature of the cosmos must explain not only the possibiity but also the actuality of rational animals: their occurrence cannot be a brute fact or cosmic accident.
I take Nagel to be maintaining that the eventual existence of some rational beings or other is no accident but is included in the nature of things from the beginning -- which is consistent with maintaining that there is an element of chance involved in the appearance of any particular instance of reason such as Beethoven. So eventually nature must produce beings capable of understanding it. We are such beings. "Each of our lives is part of the lengthy process of the universe waking up and becoming aware of itself." (85)
Nagel's thesis is not obvious. Why can't reason be a fluke? Even if we grant Nagel that the intelligibility of nature could not have been a fluke or brute fact, how does it follow that the actual existence of some rational beings or other, beings capable of 'glomming onto' the world's intelligible structure, is not a fluke? Nagel's argument needs some 'beefing up' so that it can meet this demand.
1. Let's start with the idea that nature is intelligible. Why? That the world is intelligible is a presupposition of all inquiry. The quest for understanding rests on the assumption that the world is understandable, and indeed by us. The most successful form of this quest is natural science. The success of the scientific quest is evidence that the presupposition holds and is not merely a presupposition we make. The scientific enterprise reveals to us an underlying intelligible order of things not open to perception alone, although of course the confirmation of scientific theories requires perception and the various instruments that extend it.
2. Now what explains this underlying rational order? Two possibilities. One is that nothing does: it's a brute fact. It just happens to be the case that the world is understandable by us, but it might not have been. The rational order of things underpins every explanation but itself has no explanation. The other possibility is that the rational order has an explanation, in which case it has an explanation by something distinct from it, or else is self-explanatory. On theism, the world's rational order is grounded in the divine intellect and is therefore explained by God. On what I take to be Nagel's view, the rational order is self-explanatory, a necessary feature of anything that could count as a cosmos.
Nagel views the intelligibility of the world as "itself part of the deepest explanation why things are as they are." (17). Now part of the way things are is that they are understandable by us. Given that the way things are is intelligible, it follows that the intelligibility of the world is self-explanatory or self-grounding.
Our second premise, then, is that the intelligibilty of the world is self-explanatory, hence a necessary feature of anything that could count as a cosmos.
3. Our third premise is that intelligibility is an an inherently mind-involving notion. Necessarille, then x is intelligible to some actual or possible mind. Nothing is understandable unless it is at least possible that there exist some being with the power of understanding.
The conjunction of these three premises entails the possibility of rational beings, but not the actuality of them. There would seem to be a gap in Nagel's reasoning. The world is intelligible, and its intelligibility is a necessary feature of it. From this we can infer that, necessarily, if the cosmos exists, then it is possible that there be rational beings. But that is as far as we can get with these three premises.
4. What Nagel seems to need is a principle of plenitude that allows us to pass from the possibility of rational beings to their actual existence. J. Hintikka has ascribed to Aristotle a form of the principle according to which every genuine possibility must at some time become actual. This would do the trick, but to my knowledge Nagel make no mention of any such principle.
5. I suggest that theism is in a better position when it comes to explaining how both intelligibility and mind are non-accidental. Intelligibility is grounded in the divine intellect which necessarily exists. So there must be at least one rational being. We exist contingently, but the reason in us derives from a noncontingent source.
I'm wondering if Nagel might have another option, instead of 4 or 5. Would a claim that the existence of rational beings is a fluke itself be another appeal to brute fact? If so, then given that we are rejecting brute fact as an explanation for existence itself, would it be reasonable to reject it as an explanation for the existence of rational beings?