Tipler was a big advocate of there being no other intelligent life in
the galaxy in the early 1980s (and no doubt his position has not
changed on the subject) because not one single alien has left any
evidence that it has visited Earth in its 5 billion year history. So
the logic is, if they haven't come to us, they must not exist.

Tipler's reasoning is that if even one ETI made a Von Neumann type
probe, the kind that can reproduce itself over and over as it cruises
the galaxy, then we should have been visited by one by now.

I can rattle off at least a half dozen reasons why this is NOT a reason
why ETI do not exist. Don't worry, I am not going to the other extreme
and saying that because I think Tipler is wrong in this area that ETI
therefore MUST exist, only that his particular concept does not mean
no ETI as far as I am concerned.