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

The secret of genius is to carry the spirit of childhood into maturity.

Even in the important matter of cranial capacity, Men differ more widely from one another than they do from the Apes; while the lowest Apes differ as much, in proportion, from the highest, as the latter does from Man.

A well-worn adage advises those who set out upon a great enterprise to count the cost, yet some of the greatest enterprises have succeeded because the people who undertook them did not count the cost.

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

It is the first duty of a hypothesis to be intelligible.

People never will recollect that mere learning and mere cleverness are of next to no value in life, while energy and intellectual grip, the things that are inborn and cannot be taught, are everything.