Nagel’s claim “certain things are so remarkable that they have to be explained as non-accidental if we are to pretend to a real understanding of the world” is one of those things that seems like it might be right, but turns out instead to be difficult to even make sense of. The key words are “remarkable” and “non-accidental”, and the trick is to interpret these in a way that doesn’t render Nagel’s dictum plainly false or question begging.
It sounds like Nagel is gesturing towards probabilities–”remarkable” meaning something like highly unlikely, and “non-accidental” something like necessary, or highly likely. The reviews I’ve read take him as saying as much, though taken literally this reading makes the claim hopeless. It’s precisely because highly unlikely things aren’t necessary that they are unlikely, and it’s precisely those things that don’t need any special explanation. Highly unlikely things happen all the time, and this present no special problems. If I pick one leaf on a tree and draw an outline on the ground shaped just like it, it’s highly unlikely it will land in just that spot when it falls in the Autumn. But of course each leaf falls in some leaf shaped spot which itself was just as unlikely a resting place as the one I picked. This is the kind of unlikely event that happens all the time because there is a mechanism that guarantees some comparatively unlikely event or another will happen–given gravity, the properties of leaves, and the actions of the wind it is guaranteed this leaf will fall and so come to rest in an unlikely spot. And we don’t need to render its final resting spot ‘non-accidental’ to think we understand how it got there. This kind of case is like a lottery with a guaranteed winner–it may have been wildly unlikely that I picked the winning ticket, but not that the winning ticket was picked by someone.
Trying again, maybe “non-accidental” means something more like “brought about in special or non-natural ways”, so that the claim is something to the effect that some things are so unlikely that the ordinary workings of nature cannot plausibly account for them. Some things are indeed like this. If I find in my barn a spiders’ web spelling out “some pig” I’d describe that as remarkable, and I certainly wouldn’t think it adequately explained by the chance workings of a normal spider–I’d suspect a clever prank. Unlike with the leaf case, there is no reliable non-intelligent mechanism that could produce such an unlikely result (or, to be more precise, the odds against any such mechanism producing such a result are astronomical). This kind of case is like a lottery without a guaranteed winner–if the odds against a winner are high enough, we should suspect fraud if one is produced (suppose there’s one winning ticked out of 100 million and only one ticket is pulled).
A lot of what Nagel says suggests something like this. Life, he thinks, is unlikely to the point of being not adequately explained by the workings of nature as described by physics and chemistry in the same sort of way as the talents of a spider aren’t enough to explain a legible web. In this case, yes, life needs a different kind of explanation than other things in nature. But this is unsatisfying because there’s no clear reason to suppose it’s true even if it is generally unlikely that life will arise through ordinary physical processes. For all we know the creation of life is a more like the lottery with the guaranteed winner case than the literate spider case because there are so many chances, so to speak, of life arising given the vastness of the universe and the number of planets it presumably holds. If this is Nagel’s point, it’s a rather weak one.
To do something more interesting with Nagel’s claim I think we have to take “remarkable” as pointing to something more than probabilities, and we can get at it by taking the hint of subjectivity in the word he chose more seriously. It matter to us whether things are alive or not, because it matters enormously to us that we are alive. The difference between life and non-life is of enormous salience to our ways of understanding ourselves and our place in the grand scheme of things. From our perspective this makes us look fundamentally more like trees and squirrels and squid than rocks, water, and salt. This sense that life is special, something that divides the things of the universe into two fundamentally different kinds, can suggest that it is a rather remarkable thing in a way that doesn’t involve probability. Intuitively, it seems that the most easily imagined universe would be one with only inanimate matter, so that it would take something more to add life to the picture–life takes us from a universe with one kind of thing to a universe with two kinds of things. I suspect something like this is really what is driving Nagel here–he is, intentionally or not, harkening back to the idea of a chain of being, the idea of a world with different levels of beings arranged hierarchically, so that living things are of a ‘higher’ and qualitatively different order than non-living things. It’s more remarkable that turtles exist than rocks, because a universe with turtles is ontologically richer at its most basic level–its a universe with two basic kinds of things rather than merely one. And it does seem right, if Nagel goes this far, to suppose too that it can’t be an accident that the universe is ontologically richer than it might have been.