[The following is taken from Robert Heinlein's speculative-fiction novel
 "Time Enough for Love", a story about a man with an indefinite life-span.
 In this chapter he has time-travelled back into the past to 
 meet himself and his family... and falls in love/lust with his mother].

"Sin" like "love" [is] a word hard to define.
It comes in two bitter but vastly different flavours.
The first lay in violating the taboos of your tribe.
This passion he felt was certainly sinful by the taboos of the
tribe he had been born into - incestuous in the first degree.

But it could not possibly be incest to Maureen.

To himself?
He knew that "incest" was a religious concept not a scientific one, 
and the last twenty years had washed away
in his mind almost the last trace of his tribal taboo.
What was left was no more than that breath of garlic in a good salad;
it made Maureen more enticingly forbidden (if such were possible!);
it did not scare him off.
Maureen did not *seem* to be his mother -
because she did not fit his recollection of her
either as a young woman or as an old woman.

The other meaning of "sin" was easier to define because it was not
clouded by the murky concepts of religion and taboo:
Sin is behavious that ignores the welfare of others.

Suppose he stuck around and managed somehow (stipulate safe opportunity)
to bed Maureen with her full cooperation?
Would she regret it later? Adultery?
The word meant something here.


The real question is the only one that had *ever* stopped him him
when temptation coincided with opportunity [...]
the chance of placing a congenital handicap on a child.

Transcribed for the Undesirable Propagation Unit.