Practical wisdom is only to be learned in the school of experience. Precepts and instruction are useful so far as they go, but, without the discipline of real life, they remain of the nature of theory only.Samuel Smiles
A work of art that contains theories is like an object on which the price tag has been left.Alexander Pope
All theory, dear friend, is gray, but the golden tree of life springs ever green.Johann Wolfgang von Goethe
Experience without theory is blind, but theory without experience is mere intellectual play.Immanuel Kant