Movement will cease before we are weary of being useful. - Leonardo da Vinci

When you represent in your work shadows which you can only discern with difficulty, and of which you cannot distinguish the edges so that you apprehend them confusedly, you must not make them sharp or definite lest your work should have a wooden effect.

Learning never exhaust the mind

For once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return.