Since wealth always implies an accumulated store of good things, every one comes to think of the worth of his accumulation as estimated by what it will do at any time, in any place, in satisfying any need. So, most naturally, we associate our ideas of value with trade. An exchange of horses brings in not only the present qualities of each horse for present service, but all the qualities and circumstances affecting the possibilities of disposing of the horse whenever something else is needed more. An expert judge of horses not only knows when the horse is sound in wind and limb, and what are the signs of docility, speed, etc., but also what the rest of the world considers the qualities of a satisfactory horse. In all experience peculiar circumstances, varying the relation between wants and satisfaction, affect immensely our estimate of value. When an ancient king shouted, "My kingdom for a horse!" he was doubtless moved by the uselessness of a kingdom to one about to lose his life in battle for want of a horse and also by the difficulty at that particular moment of gaining a horse.
In ordinary experience everybody estimates value by some comparison with what he can obtain in exchange. A picture of a friend may be priceless in two senses: first, of so great importance to its owner that nothing can buy it; second, of so little importance to anybody else that nobody will give anything for it. In any inventory of wealth the picture could not be counted; it is valueless. But in any judgment of personal welfare it may be beyond price. So, in the universal experience, the term value has come to be used as essentially connected with exchanges and with property as stored to meet all kinds of wants.
_Value in services._--In a similar way experience has developed the idea of value with reference to services. A service may be invaluable, as when a physician saves the life of his patient, but the value of that service is estimated by the return expected in the community where the physician and his patient live. Many services of highest usefulness and most important in welfare cannot be valued in terms of wealth, because no wealth can secure them. Love and patriotism and philanthropy cannot be had for wealth. But in comparison of two services rendered for hire, both are measured by their general utility, either directly or indirectly, and even this estimate is modified by the readiness with which either service can be secured. One who seeks the service of an artist who stands alone among ten thousand people, may be willing to give the services of a thousand other men, which can be had for a trifle, because they are everywhere abounding.