From fanaticism to barbarism is only one step.

No man has received from nature the right to command his fellow human beings.

Scepticism is the first step on the road to philosophy.

All abstract sciences are nothing but the study of relations between signs.

There are three principal means of acquiring knowledge... 

observation of nature, reflection, and experimentation. 

Observation collects facts; 

reflection combines them; 

experimentation verifies the result of that combination.