Most people like to believe something is or isn't true, but great scientists can tolerate the ambiguity.
This is actually from a talk by Richard Hamming on "why do so few scientists make significant contributions and so many are forgotten in the long run?". It’s included as the last chapter of his book The Art of Doing Science and Engineering. You can also find the full transcript of the talk at You And Your Research.
Great scientists believe the theory enough to go ahead and do the research. However, they also doubt it enough to notice the errors and faults so they can step forward and create the new replacement theory.
They are well aware of why their theories are true and where are some slight misfits that don't fit right now. Darwin writes in his autobiography that he found it necessary to write down every piece of evidence which appeared to contradict his beliefs because otherwise, they would disappear from his mind.
If we believe too much, we'll never notice the flaws. If we doubt too much, we won't get started. It requires a lovely balance.