All means are scarce, i.e., limited with respect to the ends that they could possibly serve. If the means are in unlimited abundance, then they need not serve as the object of attention of any human action. For example, air in most situations is in unlimited abundance. It is therefore not a means and is not employed as a means to the fulfillment of ends. It need not be allocated, as time is, to the satisfaction of the more important ends, since it is sufficiently abundant for all human requirements. Air, then, though indispensable, is not a means, but a general condition of human action and human welfare.
Secondly, these scarce means must be allocated by the actor to serve certain ends and leave other ends unsatisfied. This act of choice may be called economizing the means to serve the most desired ends.
- Scarcity on Wikipedia