A Letter on the Blind for the Benefit of Those Who Can See, 1749

For abstraction consists only in separating the perceptible qualities of bodies, either from other qualities, or from the bodies to which they apply. Errors arise when this separation is poorly done or wrongly applied: poorly done in philosophical questions, and wrongly applied in physical and mathematical questions. An almost sure way to err in philosophy is to fail to simplify enough the objects under study; and an infallible way to obtain defective results in physics and mathematics is to view the objects as less composite than they are.