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There can be theory but, you know, the problem is you've got to be able to test it. So theories are one thing, testing is another.

Theory not only formulates what we know but also tells us what we want to know, that is, the questions to which an answer is needed.

Theories, for me, are just about freeing your mind. It doesn't mean the theory is going to work like a scientific theory works. It's about freeing your mind and making you think a different way.

If you ask almost any of them, 'Do you stand behind your theory? Is this the answer?' I think almost everyone would say, 'No, no, no. I'm just trying to expand the range of possibilities.' We really don't know what's going on.

A theory must be tempered with reality.

It is a test of true theories not only to account for but to predict phenomena.

I don't think about a theory of everything when I do my research. And even if we knew the ultimate underlying theory, how are you going to explain the fact that we're sitting here? Solving string theory won't tell us how humanity was born.

There is no complete theory of anything.

Whenever a theory appears to you as the only possible one, take this as a sign that you have neither understood the theory nor the problem which it was intended to solve.

Every theory presented as a scientific concept is just that; it's a theory that tries to explain more about the world than previous theories have done. It is open to being challenged and to being proven incorrect.