There are people who think that everything one does with a serious face is sensible.

Man…who lives in three places – in the past, in the present, and in the future – can be unhappy if one of these three is worthless. Religion has even added a fourth – eternity.

Of all the inventions of man I doubt whether any was more easily accomplished than that of a Heaven.

The most heated defenders of a science, who cannot endure the slightest sneer at it, are commonly those who have not made very much progress in it and are secretly aware of this defect.

First there is a time when we believe everything, then for a little while we believe with discrimination, then we believe nothing whatever, and then we believe everything again - and, moreover, give reasons why we believe.

The more experiences and experiments accumulate in the exploration of nature, the more precarious the theories become. But it is not always good to discard them immediately on this account. For every hypothesis which once was sound was useful for thinking of previous phenomena in the proper interrelations and for keeping them in context. We ought to set down contradictory experiences separately, until enough have accumulated to make building a new structure worthwhile.

To do the opposite of something is also a form of imitation, namely an imitation of its opposite.