There are three principal ...

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.
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.
 Denis Diderot

More phrases

If one does not know to which port one is sailing, no wind is favorable.
What a distressing contrast there is between the radiant intelligence of the child and the feeble mentality of the average adult.
We are masters of the unsaid words, but slaves of those we let slip out.
I have always found that mercy bears richer fruits than strict justice.
A lie gets halfway around the world before the truth has a chance to get its pants on.

Quotes from the same author

Posterity for the philosopher is what the other world is for the religious man.
 Denis Diderot
If your little savage were left to himself and be allowed to retain all his ignorance, he would in time join the infant's reasoning to the grown man's passion, he would strangle his father and sleep with his mother.
 Denis Diderot
The enjoyment of freedom which could be exercised without any motivation would be the real hallmark of a maniac.
 Denis Diderot
In order to get as much fame as one's father one has to much more able than he.
 Denis Diderot
All abstract sciences are nothing but the study of relations between signs.
 Denis Diderot