The greatest of Bach’s 224 cantatas is BWV 109, ‘Ich glaube, lieber Herr, hilf meinem Unglauben’. Its subject — the title translates as Mark 9:24, ‘I believe, dear Lord, help my unbelief’ — is that strange cognitive dissonance of believing something yet not believing it at the same time. Daniel Dennett’s new book, From Bacteria to Bach and Back, is aimed at those who suffer from this intermittent unbelief, though not about God — Dennett is, after all, one of modern philosophy’s most prominent atheists — but about his specialist subject: evolution by natural selection.