A legal maxim goes, maternity is a fact, while paternity is a presumption. The law has always known this. A mother's identity is settled by the act of birth itself. A father's is, in the ordinary case, a conclusion the law draws and then trusts.
The old maxim put it plainly. The mother is certain. The father is the man whom marriage points to.
Our own evidence law carries the same idea. A child born within a valid marriage is presumed legitimate, and that presumption stands until something far stronger displaces it.
A presumption is not a fiction. It is not a guess. It is a settled conclusion that the law adopts because experience has earned it.