From "The Objectivist Ethics" -- passages I've highlighted.
Aristotle did not regard ethics as an exact science; he based his ethical system on observations of what the noble and wise men of his time chose to do, leaving unanswered the questions of: why they chose to do it, and why he evaluated them as noble and wise.
The capacity to experience pleasure or pain is innate in a man's body; it is part of his nature, part of the kind of entity he is. He has no choice about it, and he has no choice about the standard that determines what will make him experience the physical sensation of pleasure or of pain. What is that standard? His life.
The pleasure-pain mechanism in the body of man -- and in the bodies of all the living organisms that possess the faculty of consciousness -- serves as an automatic guardian of the organism's life. The physical sensation of pleasure is a signal indicating that the organism is pursuing the right course of action. The physical sensation of pain is a warning signal of danger, indicating that the organism is pursuing the wrong, course of action, that something is impairing the proper function of its body, which requires action to correct it.
Consciousness -- for those living organisms which possess it -- is the basic means of survival.
Man has no automatic code of survival. He has no automatic course of action, no automatic set of values. His senses do not tell him automatically what is good for him or evil, what will benefit his life or endanger it, what goals he should pursue and what means will achieve them, what values his life depends on, what course of action it requires. His own consciousness has to discover the answeres to all these questions -- but his consciousness will not function automatically. Man, the highest living species on this earth -- the being whose consciousness has a limitless capacity for gaining knowledge -- man is the only living entity born without any quarantee of remaining conscious at all. Man's particular distinction from all living species is the fact that his consciousness is volitional.
Psychologically, the choice "to think or not" is the choice "to focus or not." Existentially, the choice to "focus or not" is the choice "to be conscious" or not. Metaphysically, the choice "to be conscious or not" is the choice of life or death.
Consciousness -- for those living organisms which posess it -- is the basic means of survival. For man, the basic means of survival is reason. Man cannot survive, as animals do, by the guidance of mere percepts. A sensation of hunger will tell him that he needs food (if he has learned to identify it as "hunger"), but it will not tell him how to obtain his food and it will not tell him what food is good for him or poisonous. He cannot provide for his simplest need without a process of thought.
But man's responsibility goes stlll further: a process of thought is not automatic nor "instinctive" nor involuntary -- nor infallible. Man has to initiate it, to sustain it and to bear responsibility for its results.
Nothing is given to man on earth except a potential and the material on which to actualize it. The potential is a superlative machine: his consciousness; but it is a machine without a spark plug, a machine of which his own will has to be the spark plug, the self starter, and the driver; he has to discover how to use it and he has to keep it in constant action.
A being who does not know automatically what is true or false, cannot know automatically what is right or wrong, what is good for him or evil.
What, then, are the right goals for man to pursue? What are the values his survival requires? That is the question to be answered by the science of ethics And this, ladies and gentlemen, is why man needs a code of ethics.
Ethics is not a mystic fantasy -- nor a social convention --nor a dispensable, subjective luxury. Ethics is an objective, metaphysical necessity of man's survival -- not by the grace of the supernatural nor of your neighbors nor of your whims, but by the grace of reality and the nature of life.
The Objectivist ethics holds man's life as the standard of value -- and his own life as the ethical purpose of every individual man.
I'm tired of typing, so the above is an incomplete overview of the rational approach underlying the Objectivist ethics. Objective readers should get the drift in it
One caution ... there may be some typos.