His parents were writers of very dangerous books

Wittgenstein himself wrote in his notebooks a definitive, warning sentence against suicide: "If this is allowed, then everything is allowed."

When Sylvia Hughes gassed herself in Primrose Hill in February 1963, aged 30, her two children with Ted Hughes were tiny, their daughter Frieda not yet three, their son Nicholas just one.

At least she took care they should not die, too.

For any children to grow up knowing their mother chose to leave them this way, a few months after separating from their father, would be hard ultimately to survive. But Plath had also written hypnotically powerful poetry in the last months of her life that doesn't just express vengefulness but also announces and even celebrates her own death ("Dying/ Is an art, like everything else,/ I do it exceptionally well").

These Ariel poems are both unignorable and morally repugnant. Then six years later, in 1969, clearly under Plath's influence, Hughes's next partner, Assia Wevill, killed herself in the same way, murdering her four-year-old daughter, Shura, the half-sister of Frieda and Nicholas.

For many years, Hughes was pilloried by feminist critics and treated as little short of the murderer of the greatest woman poet of her time. Only in 1998, when he was dying of cancer, did he speak directly of his marriage to Plath in Birthday Letters, a loosely written confessional sequence in which he portrayed himself as helplessly taken by surprise by Plath's deadliness, uncontrollably surging up from her past, not from within their relationship.

When Hughes was posthumously awarded the Whitbread Prize for Birthday Letters in 1999, it was accepted with great ceremony by Frieda Hughes. She has courageously embraced and honoured the legacy of both her parents, writing very Hughesian poetry herself and also in 2004 editing a "restored edition" of Ariel, reinstating her mother's original selection and arrangement of the poems which had been altered by Hughes.

In her introduction she said she did not want her mother's death to be commemorated but her life - "the fact that she had existed, lived to the fullness of her ability, been happy and sad, tormented and ecstatic, and given birth to my brother and me".

Poignantly, she also noted that Plath had continued to provide for them in the form of royalties: "Through the legacy of her poetry, my mother still cared for us."

Nicholas took another path, following no literary or artistic career but leaving the country for Alaska. By specialising as a marine biologist in salmonids, he did follow one of his father's great interests (in Birthday Letters, there's a poem in which Hughes, ruefully recalling trouble with Plath, points out that he could at that moment instead have been fishing off a rock in Australia).

It has been reported that shortly before Nicholas's death he had turned to pottery for self-expression.

We cannot know how much his parents' writing contributed to Nicholas Hughes's tragic death and it would be intrusive and distasteful to speculate. But perhaps we can say that the work of both parents needs to be treated with more caution and scepticism than it sometimes has been. They are dangerous books. I remember as a sixth-former, quite ridiculously being given Plath's horror poems to recite in a school concert and being told that Hughes's book Crow, an almost insane expression of savagery, was a great work.

When I came to read Ted Hughes's Letters, I was not surprised to find that this great country man and nature-lover, who celebrated so much animal violence, admitted near the end of his life that he had never heard a nightingale.