Physical goods can fulfill a wider range of services if they are more abundant. Consequently, human beings economize them—that is, they reserve them for their highest-ranked uses, rather than wasting them unnecessarily. If a thing is present in virtually unlimited quantities, then humans do not act to gain and/or keep it; consequently, it is not technically a good. (If necessary, consult the Glossary.) A thing not available in sufficient supply to satisfy all the uses for which it is sought is said to be scarce. Thus a thing can be a praxeological good if and only if it is scarce.

For instance, Crusoe does not act to conserve or increase his supply of air, although a person stranded on the moon or trapped underwater might need to do so. Although human beings have a critical need for atmospheric oxygen (cf. p. 2.2.1, including the second Details box), air is not ordinarily a goal of human action and therefore does not qualify as either an objective or subjective value or as a good. (The issue of clean air conservation will be addressed in Section 5.)