Robert A. Heinlein - The instinct to survive is human nature itself, and every...

The instinct to survive is human nature itself, and every aspect of our personalities derives from it. Anything that conflicts with the survival instinct acts sooner or later to eliminate the individual and thereby fails to show up in future generations.... A scientifically verifiable theory of morals must be rooted in the individual's instinct to survive--and nowhere else!--and must correctly describe the hierarchy of survival, note the motivations at each level, and resolve all conflicts. We have such a theory now; we can solve any moral problem, on any level. Self-interest, love of family, duty to country, responsibility toward the human race.... The basis of all morality is duty, a concept with the same relation to group that self-interest has to individual.