The only people, scientific or other, who never make mistakes are those who do nothing.

The doctrine of transmigration… was a means of constructing a plausible vindication of the ways of the cosmos to man; … none but very hasty thinkers will reject it on the grounds of inherent absurdity.

Science commits suicide when it adopts a creed.

The only objections that have occurred to me are, 1st that you have loaded yourself with an unnecessary difficulty in adopting Natura non facit saltum so unreservedly. . . . And 2nd, it is not clear to me why, if continual physical conditions are of so little moment as you suppose, variation should occur at all. However, I must read the book two or three times more before I presume to begin picking holes.

Make up your mind to act decidedly and take the consequences. No good is ever done in this world by hesitation.

It is not to be forgotten that what we call rational grounds for our beliefs are often extremely irrational attempts to justify our instincts.

If there is anything in the world which I do firmly believe in, it is the universal validity of the law of causation.