By a strange irony, alleged Aurora mass murderer James Holmes was a doctoral student of neuroscience—the discipline that will, according to its most ardent and enthusiastic advocates, finally explain Man to himself after millennia of mystery and self-questioning.

But what could count as an explanation of what James Holmes did? At what point would we be able to say, “Aha, now I understand why he dyed his hair like the Joker and went down to the local cinema and shot all those people?” When we have sifted through his biography, examined his relationships, listened to what he has to say, and put him through all the neuropsychological and neurological tests, will we really be much wiser?

Like Anders Breivik, the young Norwegian who killed 77 people in Norway by bomb and gun, Holmes is reported to have been a “loner,” a young man without the social skills or perhaps the inclination to mix with his peers in a normal way. But such loners, though a small minority, are numbered in the thousands and tens of thousands; vanishingly few of them act like Breivik or Holmes, and many, indeed, make valuable contributions to society. Preventive detention for loners, or even special surveillance of them, would hardly be justified.

The same is true of any other characteristic that might link Breivik and Holmes to their acts. Even the presence of a recognized mental illness, such as schizophrenia, would not suffice, since most people with that affliction don’t act in this fashion. And the temptation to indulge in a circular argument, where the explanandum becomes the explanans and vice versa, must be resisted, because it offers the illusion of understanding where there is none: “He must have been mad to do this; and he did it because he was mad.”

The multifactorial analyses to which experts are inevitably driven—a bit of genetics here, a bit of parenting there, plus a dash of social pressure, culture, and the legal availability of weaponry thrown into the explanatory soup, as the weird sisters threw eye of newt and wool of bat into their cauldron—will leave us not much better off. The mesh will never be drawn fine enough for us to be able to say: “Now, at last, I understand.”

And yet our nature drives us to seek an explanation and an understanding (the two are related but not quite the same). Even if we felt like it, we cannot say: “Well, such things happen; let us hope, Inshallah, that they never happen again.” We must know the how, but also the why.

An atrocious event like the Aurora massacre brings us up sharply against something that for the most part we ignore: that, for metaphysical reasons, our explanatory reach exceeds our grasp and will do so forever. We seek a final explanation, but cannot reach one because, as Haitian peasants say, “Behind mountains, more mountains.”

Theodore Dalrymple is a contributing editor of City Journal and the Dietrich Weismann Fellow at the Manhattan Institute.