One must credit an hypothesis with all that has had to be discovered in order to demolish it.
It is theory that decides what can be observed.
There are two times in a man's life when he should not speculate: when he can't afford it, and when he can.
Theories are always very thin and insubstantial, experience only is tangible.
You must stick to your conviction, but be ready to abandon your assumptions.
Your assumptions are your windows on the world. Scrub them off every once in a while, or the light won't come in.
A speculator is a man who observes the future, and acts before it occurs.
The narrower the mind, the broader the statement.
In order to shake a hypothesis, it is sometimes not necessary to do anything more than push it as far as it will go.
I never guess. It is a shocking habit -- destructive to the logical faculty.
The temptation to form premature theories upon insufficient data is the bane of our profession.
No theory is good unless it permits, not rest, but the greatest work. No theory is good except on condition that one use it to go on beyond.
A theory is no more like a fact than a photograph is like a person.
When speculation has done its worst, two and two still make four.
Some theories are good for nothing except to be argued about.