Where our physical body automatically acts to preserve our life (to the extent that it is able) such as the heart pumping blood, or the lungs breathing air, our conceptual mind does not act automatically.
As a being of free-will, one can choose to take those actions that preserve one’s life (from eating healthy food, to engaging in rewarding relationships) or choose to engage in actions that result in one’s destruction (from consuming poison to engaging in sacrificial, destructive relationships). If one wishes to live, one must choose to act to preserve one’s life.
To choose hold one’s own life as the highest value is to be egoistic.
Self-interest (egoism) means choosing to hold one’s life as the ultimate standard of value (the measure by which one determines and judges one’s actions), regarding oneself as the primary beneficiary of one’s actions. Such a standard determines what actions are good (that which preserves and sustains one’s life) and that which is evil (that which harms and destroys one’s life).
However, egoism is not an out-of-context concept but to be valid (i.e., truly egoistic and not self-destructive) egoism must be guided not by whims, an act of faith, or unanalyzed emotions, but by reason.
Reason, according to Ayn Rand, is one’s means of conceptually grasping reality based on the evidence of the senses (facts). It is a human being’s means of understanding reality and determining how to act in it. Survival for human beings is not automatic. Unlike other organisms — bacteria, elephants, and apes — humans must alter their environment to survive. One must use their reason to rearrange the objects in their environment to create the values — shelter, food, books, energy, medicine — one requires to survive. Reason is a human being’s distinctive means of survival.
To be rational, as philosopher Ayn Rand has observed, is to accept reason as one’s only source of knowledge, and as one’s only guide to action (as opposed to going by one’s emotions, whims, or on faith). Such intellectual independence does not mean that one cannot learn from others, but that such second-hand knowledge must still be processed and validated by one’s mind. Whether one is alone on a desert island or living in modern society: the individual must think logically and act rationally if life is their goal.
Rational self-interest means choosing to take those actions required by a human being, according to a reality-based, objectively validated moral code of values, so that one may live a flourishing life on earth.
Self-interest indicates the end (the self) of one’s actions; rational indicates the means (reason) of obtaining that end. What objectively bridges the two is a rational code of moral principles — virtues — that serve as one’s guide to action.