Human beings then are "rational" entities - rational as characterizing behavior that purposefully chooses means to achieve ends, with those choices being the result of conscious thought and reasoning, as contrasted with biological organisms that function almost entirely on hard-coded instinct.

To this we can add that all men will act in favor of their own individual evaluation at any given moment, of what will best serve that which they personally value the most at that point in time, which most often will be what they evaluate to be what will maximize their own personal well-being at that moment or in the future.

Of course man is a physical entity with a physical body that can only be sustained by a long series of meaningful actions, stretching from birth to death.

If man does not manage to obtain food, shelter, and physical comforts of various kinds, he will die.  The continued existence of the individual and the species is entirely dependent upon the physical environment surrounding him.

It is highly unlikely that anyone can take one action or a few actions and by this process obtain the necessities much less the luxuries as he or she will need for their entire life.  Indeed, human beings must employ their energies and take purposeful action throughout their lives so that their life can be sustained.

In this process of survival, man plays two essentially contradictory and opposed roles.  He is at the same time both a consumer, and yet also a producer.

That human beings are consumers are readily apparent.  Life demands consumption.  If consumption ceases totally, life ceases.  Acting man is consuming man.  Man consumes in order to live.