A discussion between Haldane and a friend began to take a predictable turn. The friend said with a sigh, 'It's no use going on. I know what you will say next, and I know what you will do next.' The distinguished scientist promptly sat down on the floor, turned two back somersaults, and returned to his seat. 'There,' he said with a smile. 'That's to prove that you're not always right.'

Man armed with science is like a baby with a box of matches.

Would I lay down my life to save my brother? No, but I would to save two brothers or eight cousins.

Teleology is like a mistress to a biologist: he cannot live without her but he's unwilling to be seen with her in public.

I do not believe in the commercial possibility of induced radioactivity.

[Children] are taught that it is a virtue to accept statements without adequate evidence, which leaves them a prey to quacks of every kind in later life, and makes it very difficult for them to accept the methods of thought which are successful in science.

It was a reaction from the old idea of "protoplasm", a name which was a mere repository of ignorance.