Several subtleties in the concept of force should be noted:
- Since force is a species of human action, the absence of an action cannot by itself constitute an instance of force. For example, if Friday asks Crusoe to build him a tree house and Crusoe declines, Friday cannot legitimately describe Crusoe's non-action as a kind of "force." As was discussed in Section 1 (see p. 1.4:7, including the "Details" box), the distinction between the existence and the nonexistence of a thing or an action is absolutely essential to our proper understanding of reality.
- Although Ayn Rand remarks that "physical force" is barred from a free-market society (
), "force" in this context should properly be distinguished from "force" as the term is understood by physicists (i. e., mass times acceleration). A masseur uses physical force, but if he practices on willing clients he is not using praxeological force. Force in this latter sense—and therefore the concept of a free market—can be accurately understood only by reference to the praxeological ideas of subjective value and value scales. 
