people a fact accepted on authority. So is the existence of Napoleon or Julius Cæsar. Familiar astronomical facts are known only in the same way, except by those who have studied astronomy. It is obvious that every one's knowledge would be very limited indeed, if we were not justified in accepting facts on the authority of others.
But we are justified only under one condition. The facts which we can safely accept must be capable of demonstration or verification. The examples I have given belong to this class. The boy can verify when he goes to France or is able to read a French book that the facts which he took on authority are true. I am confronted every day with evidence which proves to me that, if I took the trouble, I could verify the existence of Calcutta for myself. I cannot convince myself in this way of the existence of Napoleon, but if I have doubts about it, a simple process of reasoning shows me that there are hosts of facts which are incompatible with his non-existence. I have no doubt that the earth is some 93 millions of miles distant from the sun, because all astronomers agree that it has been demonstrated, and their agreement is only explicable on the supposition that this has been demonstrated and that, if I took the trouble to work out the calculation, I should reach the same result.
