No theory is good except on condition that one use it to go beyond.

Your theory is crazy, but it's not crazy enough to be true.

Professors in every branch of the sciences prefer their own theories to truth; the reason is that their theories are private property, but truth is common stock.

It is a capital mistake to theorize before one has data.

The astonishment of life is the absence of any appearances of reconciliation between the theory and the practice of life.

In theory, there is nothing to hinder our following what we are taught, but in life there are many things to draw us aside.

To be sure, theory is useful. But without warmth of heart and without love it bruises the very ones it claims to save.

All theory, my friend, is grey, But green is life's glad golden tree.

It is so much easier to talk of poverty than to think of the poor, to argue the rights of capital than to see its results. Pretty soon we come to think of the theories and abstract ideas as things in themselves.

For every fact there is an infinity of hypotheses. The more you look the more you see.