Every failure is a step to success.

It is a test of true theories 

not only to account for 

but to predict phenomena.

Fundamental ideas are not a consequence of experience, 

but a result of the particular constitution and activity of the mind, 

which is independent of all experience in its origin, 

though constantly combined with experience in its exercise.

The system becomes more coherent as it is further extended. 

The elements which we require for explaining a new class of facts 

are already contained in our system. 

In false theories, the contrary is the case.

We cannot observe external things without some degree of thought; 

nor can we reflect upon our thoughts, 

without being influenced in the course of our reflection 

by the things which we have observed.