Rothbard continues the Crusoe analogy. He covers subjectivity of value, and the concept of marginal utility.

Starting with Crusoe economics, Rothbard builds the economic concepts which can be developed by this analogy.These concepts are the axiom of human action. Among them are: man acts, man acts by virtue of his existence, man acts with purposeful behavior,...

Costs are always ex ante. There are no such things as social costs or social benefits. Costs are determined by how much entrepreneurs think consumers will pay. Costs are not determined by supply and demand. Nobody waits for costs to raise prices