We want to have certainties and no doubts — results and no experiments — without even seeing that certainties can arise only through doubt and results only through experiment.

We only control what we don't trust.

Avoid the precepts of those thinkers whose reasoning is not confirmed by experience.

we never experiment with just one electron or atom or (small) molecule. In thought-experiments we sometimes assume that we do.

We must never assume that which is incapable of proof.

You can’t experiment with tradition.

We would not dare to conceive the things which are really mere commonplaces of existence.

We cannot expect to gain true knowledge without acting upon our ideas.

Neither in the arts, nor in logic, nor in life should an idea by in any way treated as a thing.

He had disciplined his mind not to speculate; speculation in advance of the facts was always futile and could be dangerous.

Let us not make random conjectures about the greatest matters.

In any case, we do not advance the human cause by refusing to consider ideas that make us frightened.

We ought not to desire the impossible.

Never let us utter what we never can know,
And chiefly when it works another's woe.