A man may imagine things that are false, but he can only understand things that are true, for if the things be false, the apprehension of them is not understanding. 
Errors are not in the art but in the artificers.
I can calculate the motion of heavenly bodies, but not the madness of people.
I do not know what I may appear to the world; but to myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.
If I have done the public any service, it is due to my patient thought.
If I have seen further than others, it is by standing upon the shoulders of giants.
Tact is the art of making a point without making an enemy.
To every action there is always opposed an equal reaction.
We are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances.
We build too many walls and not enough bridges.