And there are the Keynesian and other under-consumptionist fallacies, which, at bottom, are just as absurd as thinking that Crusoe could become richer in consumers' goods by consuming more in order to stimulate his own production. There is a great deal that Mr. Crusoe could teach Mr. Krugman, if the arch-Keynesian would only deign to spend some quality time with him.
As the great Frederic Bastiat wrote, "How happy will nations be when they see clearly how and why what we find false and what we find true of man in isolation continue to be false or true of man in society!"

"The most important "mental equipment" for thinking about mankind is the "category of teleology/action": our inbuilt conception of purpose and purposeful behavior. Action (purposeful behavior) is the use of means to seek an end (the "end" always being "the relief from a felt uneasiness").

Just as the natural world would be a meaningless jumble of sensations without the category of causality, the social world around us (as well as our understanding of our own states of mind) would be a meaningless jumble of sensations if we were not equipped with the category of teleology/action. The conception of action in general is necessarily prior to the cognition of any particular action.
Gestures and words would be meaningless motions and sounds unless the mind were able to apply the conception of "purpose" to them. And how could you explain "purpose" to someone who did not already have an inbuilt conception of it, when such an explanation would be, to your "student," meaningless motions and sounds?

If I know anything, I know what action is. With every sentence I write with the purpose of communicating to you, the reader, I live it. And if you are looking at these words with the purpose of understanding them, you, too, are living it. Should I try to deny the reality of purpose, means and ends, I would find myself caught in the manifest absurdity of trying to deny the reality of trying."