The sentence by Leonardo da Vinci:

He who loves practice without theory is like the sailor who boards ship           
without a rudder and compass and never knows where he may cast.        

Several sentences by Albert Einstein:

Everything that is really great and inspiring is created
by the individual who can labor in freedom.

Most of the fundamental ideas of science are essentially simple,
and may, as a rule, be expressed in a language comprehensible
to everyone.

Everything should be made as simple as possible, but not simpler.

The search for truth is more precious than its possession.

The truth of a theory is in your mind, not in your eyes.

Imagination is more important than knowledge.

Learn from yesterday, live for today, hope for tomorrow. The important thing is to not stop questioning.

Peace cannot be kept by force. It can only be achieved through understanding.