Whether and under what conditions a thing is useful to me, whether and under what conditions it is a good, whether and under what conditions it is an economic good, whether and under what conditions it possesses value for me and how large the measure of this value is for me, whether and under what conditions an economic change of goods will take place between two economizing individuals, and the limits within which a price can be established if an exchange does occur... Economic theory is concerned ... with the conditions under which men engage in provident activity directed to the satisfaction of their needs.