Three from Jose Ortega y Gasset:
"A revolution only lasts fifteen years, a period which coincides with the effectiveness of a generation."
"Effort is only effort when it hurts."
"The metaphor is perhaps one of man's most fruitful potentialities. Its efficacy verges on magic, and it seems a tool for creation which God forgot inside one of His creatures when He made him."
A note on Ortega: Many of us read his Revolt of the Masses in college, but not everyone knows that this philosopher also wrote one of the all-time classics on the joys and ethics of hunting, Meditations on Hunting.