He marvelled at the fact that the cats had two holes cut in their fur at precisely the spot where their eyes were.

If an angel were to tell us about his philosophy, I believe many of his statements might well sound like '2 x 2= 13'.

If we make a couple of discoveries here and there we need not believe things will go like this for ever. An acrobat can leap higher than a farm-hand, and one acrobat higher than another, yet the height no man can overleap is still very low. Just as we hit water when we dig in the earth, so we discover the incomprehensible sooner or later.

The construction of the universe is certainly very much easier to explain than is that of the plant.

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.

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.

The natural scientists of the previous age knew less than we do and believed they were very close to the goal: we have taken very great steps in its direction and now discover we are still very far away from it. With the most rational philosophers an increase in their knowledge is always attended by an increased conviction of their ignorance.

We have to believe that everything has a cause, as the spider spins its web in order to catch flies. But it does this before it knows there are such things as flies.