"Unquill wondered at the implications of that.

Preventing the destruction of the human race would mean that he, at various points in the past, would never have seen it.

He then would never have taken steps to avert it.

If such steps weren't taken, then the destruction would happen anyway, which would mean . . .

Paradoxes hadn't been his specialty."
The Smallest Giant (Children of Time 1) - G.J. Winters

The Smallest Giant- Children of Time, Part 1