According to Thomas the soul is not matter, not even incorporeal or spiritual matter. If it were, it would not be able to understand universals, which are immaterial. A receiver receives things according to the receiver's own nature, so for soul (receiver) to understand (receive) universals, it must have the same nature as universals. Yet, any substance that understands universals may not be a matter-form composite. So, humans have rational souls, which are abstract forms independent of the body. But a human being is one existing, single material substance that comes from body and soul: that is what Thomas means when he writes that "something one in nature can be formed from an intellectual substance and a body", and "a thing one in nature does not result from two permanent entities unless one has the character of substantial form and the other of matter." Economics Thomas Aquinas addressed most economic questions within the framework of justice, which he contended was the highest of the moral virtues. He says that justice is "a habit whereby man renders to each his due by a constant and perpetual will." He argued that this concept of justice has its roots in natural law. Joseph Schumpeter, in his History of Economic Analysis, concluded that "All the economic questions put together matters less to him than did the smallest point of theological or philosophical doctrine, and it is only where economic phenomena raise questions of moral theology that he touches upon them at all." Thomas Aquinas was careful to distinguish the just, or natural, price of a good from that price which manipulates another party. He determines the just price from a number of things. First, the just price must be relative to the worth of the good. Thomas held that the price of a good measures its quality: "the quality of a thing that comes into human use is measured by the price given for it." He goes on to say that the price of a good, measured by its worth, is determined by its usefulness to man. This worth is subjective because each good has a different level of usefulness to every man. Thomas argued, then, that the price should reflect the current value of a good according to its usefulness to man. He continues: "Gold and silver are costly not only on account of the usefulness of the vessels and other like things