You must always be puzzled by mental illness. The thing I would dread most, if I became mentally ill, would be your adopting a common sense attitude; that you could take it for granted that I was deluded.

A wheel that can be turned though nothing else moves with it, is not a part of the mechanism.

If you tried to doubt everything you would not get as far as doubting anything. The game of doubting itself presupposes certainty.

We could present spatially an atomic fact which contradicted the laws of physics, but not one which contradicted the laws of geometry.

That which cannot be said must not be said. That which cannot be said, one must be silent thereof.

Logic must look after itself. In a certain sense, we cannot make mistakes in logic.

Philosophy is like trying to open a safe with a combination lock: each little adjustment of the dials seems to achieve nothing, only when everything is in place does the door open.