We laugh at the Indian philosopher, who to account for the support of the earth, contrived the hypothesis of a huge elephant, and to support the elephant, a huge tortoise. If we will candidly confess the truth, we know as little of the operation of the nerves, as he did of the manner in which the earth is supported: and our hypothesis about animal spirits, or about the tension and vibrations of the nerves, are as like to be true, as his about the support of the earth. His elephant was a hypothesis, and our hypotheses are elephants. Every theory in philosophy, which is built on pure conjecture, is an elephant; and every theory that is supported partly by fact, and partly by conjecture, is like Nebuchadnezzar's image, whose feet were partly of iron, and partly of clay.