Like the term "value" (cf. p. 1.3:55), to which it is closely related, the term "cost" may refer either to an attribute or to a thing or condition having that attribute. Consequently, we may differentiate between subjective and physical costs. In the last stage of Crusoe's production, the physical costs of the tree house are the specific factors expended: the treetop, the bamboo poles, Crusoe's house-building labor, and the thatch. The subjective cost is the potential usefulness for alternative purposes of all four of these factors considered in combination, which Crusoe must forfeit in order to obtain the tree house. How useful these factors are to Crusoe, of course, depends on his specific purposes. Consequently, subjective utility is not an intrinsic property of the treetop and other factors; rather, this attribute arises from the relationship between these things and Crusoe's purposes. Crusoe proceeds with this stage of production only because the tree house's expected subjective utility exceeds its subjective cost, thus netting him a gain in utility.