Truth, lies and the mystery of Praia da Luz

The truth is: we simply don't know. This sudden lurch from the previously secure theory of a callous abduction by a stranger to a theory of a different kind of darkness reminds us just how little we know but how anxious we are for certain knowledge.

One author was extraordinarily prescient about this divided instinct: to believe the best and worst of people and feel a permanent conflict between the two.

Check out, if you haven't, Heinrich von Kleist (Penguin Classic, in translation), a German short-story writer of the late 18th century but well ahead of his time in his studies of doubt, duplicity, subconscious and the way the mind and emotions can tell different and conflicting truths - leaving us tangled about what to believe.

Kleist is often described as the first truly modern author, because he defines the condition of post-Enlightenment man with his unyielding emphasis on knowledge and rationality, suddenly undermined by unexpected evidence or shifts of perception.

The McCanns' story likewise pits our strong instincts about the vast unlikelihood of stable parents being implicated in the death of their child. It "feels" wrong. Then a fact - or apparent fact - emerges to upend our previous conviction. What are we to believe?

Kleist plays repeatedly with these motifs. His play Amphitryon tests a loyal wife against her husband's evidence that she has been unfaithful - which in "fact" she has, because a god has slipped into her husband's mortal form for the night (stuff happens).

One of his most famous phrases - "the fragile construction of the world" - conveys that the physical world and its manifestations can be treacherous when we least expect it. We are programmed to believe - but let down when we trust. The human condition is about a quest for certainty, undermined by the fallibility of the senses and intellect.

At the end of the Marquess of O, a tale of rape, subconscious, class and suspicion of other nationalities, all this is "resolved" subversively when the heroine marries the man who raped her while she was unconscious and leaves us with the enigmatic remark: "He could not have appeared to me to be a devil, had he not appeared to me as an angel first."

Angels and devils, truths and untruths - a desire to believe the best and a fear of the worst. We live with them in a world both fragile and brutal, from the feudal world of Kleist to Praia da Luz. Mere forensics can't help us with that.