In response to my query, Ayaz makes an interesting assertion:
There definitely are huge areas of human life over which reason can have no say. Reason is value-neutral and necessarily hypothetical. Reason essentially boils down to definitions. It can say if x is true, then not x must be false. But it can never provide reasons in themselves.
Reason might tell you that if you don't want to hurt others then don't do x,y and z and that's how secular ethics largely function. But as to why we shouldn't hurt others. Well we basically just assume that on trust/faith/instinct/basic principle.
It doesn't seem a huge problem as most of us agree on those basic tenants, but it's unsettling nonetheless that when the odd psychopath comes along we can't actually use reason to demonstrate why he's wrong, all we can do is disapprove.
I disagree here on a couple of points. First of all, I think we have to disambiguate logic from reason. I think what Ayaz defines as "reason", i.e. hypothetical and concerned with definitions, would be better characterized as logic. Reason, on the other hand, consists of a certain set of logical operations and modes of inquiry based in some sense* on perceptual facts. (If you prefer a different disambiguating term, e.g. science, that's fine. I'm not too hung up on terminology.) Logic is very broad, and includes kinds of operations that do not appear immediately relevant (e.g. the Banach-Tarski "paradox") to the real, physical world and the world of perception. Reason consists of particular kinds of logic and its application to draw conclusions about the real world using perceptions as — in some sense — a "foundation."
*I say "in some sense" because one particular sense, naive empiricism, which holds perceptual facts as specifically logical axioms from which we can draw formal deductions, does not appear viable.
This view of "reason" entails a deep sense in which reason is not just value-neutral but also fact-neutral. Reason doesn't tell us what we "ought" to perceive; we just use reason to draw conclusions about what the world has to be like based on what we actually perceive. Reason would work equally well, at least at the most fundamental level, even with an entirely different body of facts. (Based on earlier exercises of reason on perceptual facts, we tend to incorporate a lot of older conclusions (causality, for example) directly into our methodology (sometimes to our detriment). But there's a big difference between a fundamental metaphysical principle and merely taking something for granted because it's so well-established.)
We can say the same thing about the "value-neutrality" of reason. We can take values as (at some level) brute facts, just as our perceptual experiences are brute facts. They are simply properties of how individual human beings actually are. (Again, I say "at some level" because the supposed "brute facts" of subjective values are probably reducible to more fundamental brute facts of neurological operation. But presumably there are still some brute facts somewhere.)
We just are what we happen to be, that there's no fundamental "reason" why we "ought to be" some particular way. There's a causal story — we have a long biological evolutionary history, and a somewhat shorter social evolutionary history — but no teleological story. We are who we are because we happened to evolve this way; we have tried a minuscule fraction of the stupendous number of ways of living, and what we have today is what wasn't selected against of the ways we've tried. We could have by accident and happenstance turned out very differently. But we have had the accidents we've had and not others; physical law has exerted its selective forces; so here we are.
I've never felt at all unsettled or disturbed by this story. It doesn't bother me in the least that I don't have a teleological reason why the "psychopath" is wrong; I'm perfectly happy basing my response directly on my own and others' disapproval. To be honest, I don't really understand, at least not internally, why any atheist would feel such discomfort; I have to take them at their word. My only response is that feeling uncomfortable without a teleological ethical narrative is no more probative than feeling uncomfortable that the physical world might not be the product of a deity's intentional design. Reason is reason, and the conclusion that our values are just complicated properties of our complicated, evolved brains seems inescapable. Simply wishing the world were more in line with our emotional prejudices doesn't change the world. And I think Ayaz wants to look outside reason for a moral narrative precisely because he doesn't like the moral narrative that reason does give us. There isn't any way the world — and human society is part of the world — "ought to be"; there is only the world as it is, and what we as physical, finite, limited, evolved creatures make of it.