Nature furnishes freely and without trouble to man many of the things that he needs. She furnishes him air to breathe, water to drink, and daylight in which to work or play; but there is another class of needs which nature supplies only on the condition that man cooperates with her efforts. Man must have food, for example, and this he can have in sufficient quantity to satisfy his need only if he is willing to assist nature in producing it. Clothing and shelter and a great many other things are to be had only on the same terms. Food, clothing, shelter, and other desirable things of which the supply is limited are called wealth, provided that they may be owned and that they are external to the owner. Thus there are four characteristics of wealth. A thing, to be wealth, must be desirable, limited in quantity with respect to the demand for it, appropriable, and external to the owner. The word "economic" has several meanings one of which is relating to wealth. Economic activities, therefore, are those activities which relate to wealth, or in other words, those activities concerned with things which men desire, things relatively limited in quantity, appropriable, and external to the owner. Economics or political economy is the science of economic activities.