“According to that,” you say, “you would claim that you are under no obligation to your physician beyond his paltry fee, nor to your teacher, because you have paid him some money. Yet for all these we have great affection, great respect.” The answer to this is that the price paid for some things does not represent their value. You pay a physician for what is invaluable, life and good health, a teacher of the liberal sciences for the training of a gentleman and cultivation of the mind. Consequently the money paid to these is the price, not of their gift, but of their devotion in serving us, in putting aside their own interests and giving their time to us; they get paid, not for their worth, but for their trouble. Yet I might more truly make another statement, which I shall at once present, having first pointed out how your quibble can be refuted. “If,” you say, “the value of some things is greater than the price they cost, then, although you have paid for them, you still owe me something besides.” But, in the first place, what difference does it make what they are really worth, since the seller and the buyer have agreed upon their price? In the second place, I bought the thing, not at its own value, but at your price. “It is,” you retort, “worth more than it costs.” Yes, but it could not have been sold for more. Besides, the price of everything varies with circumstances; though you have well praised your wares, they are worth only the highest price at which they can be sold; a man, therefore, who buys them cheap, owes nothing more to the seller. Again, even if they are worth more, nevertheless the fact that their price is determined, not by their utility or efficacy, but by the customary rate of the market, does not imply that there is any gift on your part. At what would you value the service of the man who crosses the seas, and, when he has lost sight of the land, traces an unerring course through the midst of the waves, who forecasts coming storms, and suddenly orders the crew, when they have no sense of danger, to furl the sails, to lower the tackle, and to stand ready to meet the assault and sudden fury of the storm? Yet this man’s reward for such great service is paid by the passenger’s fare! What value do you set on finding lodging in a wilderness, a shelter in rain, a warm bath or a fire in cold weather? Yet I know at what price I can obtain these things when I enter an inn! How great a service does he do us who props up our tottering house, and with unbelievable skill keeps erect a group of buildings that are showing cracks at the bottom! Yet a contract for underpinning is made at a fixed and cheap rate. The city wall provides us protection from the enemy and from the sudden attacks of brigands; yet it is well known how much a workman is paid each day for erecting the towers provided with parapets to assure the public safety.