The greatest and noblest conceptions have no image wrought plainly for human vision, which he who wishes to satisfy the mind of the inquirer can apply to some one of his senses and by mere exhibition satisfy the mind. We must therefore endeavor by practice to acquire the power of giving and understanding a rational definition of each one of them; for immaterial things, which are the noblest and greatest, can be exhibited by reason only, and it is for their sake that all we are saying is said. But it is always easier to practice in small matters than in greater ones.