"Everything degenerates in the hands of man. He forces one soil to nourish the products of another, one tree to bear the fruit of another. He mixes and confuses the climates, the elements, the seasons. He mutilates his dog, his horse, his slave. He turns everything upside down; he disfigures everything; he loves deformity, monsters. He wants nothing as nature made it, not even man; for him, man must be trained like a school horse; man must be fashioned in keeping with his fancy like a tree in his garden.
Were he not to do this, however, everything would go even worse, and our species does not admit of being formed halfway."
-Jean-Jacques Rousseau (1712-1778)
from Emile or On Education
Another that I love:
"What must be done to prevent anything from being done. When it is only a question of going against the wind, one tacks. But if the sea is heavy and one wants to stand still, one must cast anchor. Take care, young pilot, for fear that your cable run or your anchor drag and that the vessel drift without your noticing."
"The man who has lived the most is not he who has counted the most years but he who has most felt life."
"How rapid is our journey on this earth! The first quarter of life has been lived before one knows the use of it. the last quarter is lived when one has ceased to enjoy it."