Every sentence I utter must be understood not as an affirmation, but as a question.
Prediction is difficult, especially the future.
The opposite of a fact is falsehood, but the opposite of one profound truth may very well be another profound truth.
And anyone who thinks they can talk about quantum theory without feeling dizzy hasn't yet understood the first thing about it.
One must always do what one really cannot.
The old saying of the two kinds of truth. To the one kind belongs statements so simple and clear that the opposite assertion obviously could not be defended. The other kind, the so-called 'deep truths', are statements in which the opposite also contains deep truth.