Time travel may be achieved one day, or it may not. But if it is, it should not require any fundamental change in world-view, at least for those who broadly share the world view I am presenting in this book.

The whole [scientific] process resembles biological evolution. A problem is like an ecological niche, and a theory is like a gene or a species which is being tested for viability in that niche.

If you reject the infinite, you are stuck with the finite, and the finite is parochial... the best explanation of anything eventually involves universality, and therefore infinity. The reach of explanations cannot be limited by fiat.

Where we have good, testable explanations, they then have to be tested, and we drop the ones that fail the tests.

The quantum theory of parallel universes is not the problem, it is the solution. It is not some troublesome, optional interpretation emerging from arcane theoretical considerations. It is the explanation - the only one that is tenable - of a remarkable and counter-intuitive reality.

Like every other destruction of optimism, whether in a whole civilisation or in a single individual, these must have been unspeakable catastrophes for those who had dared to expect progress. But we should feel more than sympathy for those people. We should take it personally. For if any of those earlier experiments in optimism had succeeded, our species would be exploring the stars by now, and you and I would be immortal.

The overwhelming majority of theories are rejected because they contain bad explanations, not because they fail experimental tests.