The rung of a ladder was never meant to rest upon.

The whole analogy of natural operations furnishes so complete and crushing an argument against the intervention of any but what are termed secondary causes, in the production of all the phenomena of the universe; that, in view of the intimate relations between Man and the rest of the living world; and between the forces exerted by the latter and all other forces, I can see no excuse for doubting that all are co-ordinated terms of Nature's great progression, from the formless to the formed from the inorganic to the organic from blind force to conscious intellect and will.

Mathematics may be compared to a mill of exquisite workmanship, which grinds your stuff to any degree of fineness.

I am too much of a sceptic to deny the possibility of anything...

A man's worst difficulties begin when he is able to do as he likes.

I have endeavoured to show that no absolute structural line of demarcation, wider than that between the animals which immediately succeed us in the scale, can be drawn between the animal world and ourselves; and I may add the expression of my belief that the attempt to draw a physical distinction is equally futile, and that even the highest faculties of feeling and of intellect begin to germinate in lower forms of life.

The rules of the game are what we call the laws of nature.