When a particular object conditions some satisfaction of ours, it possesses a superior power over our welfare to that possessed by a good which, while it affords us satisfaction, does not condition it. It would be a waste of energy to spend it in producing or conserving the thing of inferior power. The ultimate reason why our minds instinctively assign a superior importance to the things of higher power over our welfare is that such instincts contribute to our fitness in the struggle for existence. The things of lower rank possess merely utility. The things of higher rank possess a superior power which we recognize and distinguish as _value_. Lack of this distinction would lead to a waste of energy in the modifications which we effect in our environment.[218] 4. Let us consider now the essential features of the utility theory of value. According to this doctrine the sole source of value is human satisfaction. The thought is well conveyed, though the expression may be somewhat loose, by saying that the only inherently valuable thing is satisfaction, and that the value of satisfaction is passed out to, or attributed to, any external object upon which the satisfaction is dependent. Human satisfactions are quantities, for they are capable of being _more_ or _less_. But they are inexact and wavering quantities. Similarly, the esteem values of goods, derived from the satisfactions conditioned by these goods, are quantitative, but are incapable of exact or constant numerical expression. The exchange values of goods are exact and relatively constant quantities. Nevertheless, exchange values are assuredly founded upon esteem values. The description of the process by which these wavering and more or less uncertain esteem values determine the exact and definite exchange values of goods, is perhaps the most difficult part of the utility theory. But without this part—commonly called the “theory of price”—the utility doctrine remains a mere torso.