In these pages, we meet Sir Roger Penrose, who in 1970 with Stephen Hawking showed that the Big Bang "must have been a singularity" - a self-contained event with no deterministic cause. We hear from Nobel physics laureate Steven Weinberg and novelist John Updike, and theologian Richard Swinburne. The subjects share a curiosity, a quality of engagement, in the face of everything they cannot know.

What Holt is asking, after all, is unanswerable, which means that any response, even the most nuanced, must be conditional in the most fundamental sense. For Holt, any universe, even one with no beginning, has an internal logic of its own. He makes the point succinctly from the outset: "The idea that the world somehow holds the key to its own existence - and hence that it exists, necessarily, not as an accident - jibes with the thinking of some metaphysically inclined physicists, such as … Penrose, and the late John Archibald Wheeler [who coined the term black hole]."

And yet, because this key necessarily eludes us, Holt also takes a perverse pleasure in doubling back, raising questions and then poking holes in them at once.

"Even today," he writes, "when we ask why there is something rather than nothing at all, we are, wittingly or not, heirs to a way of thinking that is a vestige of early Judeo-Christianity."

Throughout Holt's book the idea of a universe built on good or perfection is still in play.

We're talking about God here, although as you'd expect that takes as many forms as there are sources in the book. For Penrose, it's a matter of Platonic ideals, which he believes are the building blocks of the universe.

But even Swinburne concedes he can find no explanation for God, whose existence, compared to the stark simplicity of Nothingness, strikes him as "vastly improbable". So where does this leave us, Holt wonders? Is cosmology just a parlour game, in which our only choice is to take a leap of faith?

The most compelling answer comes from Weinberg. The trouble with cosmological conjectures, he tells Holt, "is that we have no way, at present, of deciding whether they're true or not. It's not just that we don't have the observational data - we don't even have the theory".

Even a so-called final theory may never answer the question of why. "The more the universe seems comprehensible," Weinberg has written, "the more it also seems pointless." This is the paradox, not only for Weinberg but for Holt too.