Axiom of Action

The action axiom can be stated as follows:
“Human beings engage in purposive behaviour—i.e., they choose which scarce means are to be more fruitfully (or economically, or rationally) employed in order to satisfy their most preferred ends. This behaviour — stemming from human free will — is what we call action. As long as means are scarce and wants are not fully satisfied, human beings will keep on intentionally (or purposefully) acting.”

Why is this an axiom?

You cannot disprove it without either conceding its truth or incurring a self-contradiction.
In fact, anyone trying to disprove the action axiom would indeed engage in purposive behaviour i.e., he would be employing scarce means (his time, his intellectual labor, etc.) in order to achieve a preferred end (trying to disprove the action axiom instead of, say, watching TV or reading).
Therefore, the action axiom denier would either contradict himself — claiming the untruth of a statement he is instead performatively proving to be true — or be forced to concede the truth of the axiom itself — because otherwise he could not maintain to be acting in the sense we defined above.