In order to understand natural human ethics we must consider its natural source, that is to say, the origin of the sense of duty or social conscience.
The sense of duty is, as an inclination, inborn, and therefore hereditary. It can indeed be developed or dulled by education, but it cannot be acquired; and only diseases of the brain can destroy it where it once clearly exists. What is actually inculcated or acquired, as the case may be, is not the conscience, but the object towards which it is directed, as is the case with the feeling of shame or modesty. Just as the European woman is ashamed to exhibit her bare legs, but not her face, while with the Turkish woman the reverse holds true, so the objects of the conscience, according to acquired local customs, can be absolutely opposed to one another, or at least very different in their nature. They have, however, for the most part certain features in common, which are suited to the requirements of human nature. The reason for this we shall see below.
From what does conscience, or the sense of duty, arise? First of all from a conflict between two groups of instinctive emotions allied with instinctive impulses: (1) the group of so-called egoistic feelings and impulses, directed towards self-preservation and self-gratification; and (2) the group of sympathetic or altruistic impulses directed towards the preservation and well-being of others.
If I feel sympathy or love for a person, an animal, or an object, I suffer personally and feel displeasure as soon as the object of my sympathy suffers or is endangered. Hence the words compassion and sympathy (suffering with). I therefore seek to help the object of my sympathy, to save him even at the risk of personal injury; and thence the conflict arises. If my egotism triumphs I do not come to his aid, or at most only do so if I risk nothing thereby. If, on the other hand, my sense of sympathy is victorious, I sacrifice myself.
In the former instance I experience a feeling of dissatisfaction, the feeling of neglected duty and of remorse; in the latter I have the pleasurable sensation of duty fulfilled. And yet the nature of the object matters little. Only the intensity of the sympathy, together with the individual development of the conscience, determine the intensity of the sense of duty in any given case. An insane person can feel the most vehement sense of duty or remorse without any real object, or as the result of entirely perverted conceptions.
As every living creature, particularly if it possesses a separate nervous system, has the instinct of self-preservation, the conscience therefore results directly from the conflict between this instinct and the secondary emotions of altruistic sympathy. These latter are of later origin, and have for the most part been evolved from the attraction between the sexes (sexual love), or from the relationship of parents to the offspring dependent upon them (parental love).
The first feelings of duty and of sympathy in the animal kingdom are therefore confined to the family, and adapted to the preservation of the species. They are also exclusive, and may only persist for a short time (as in the case of cats), but frequently they are of lifelong duration. The conjugal fidelity of certain apes and parrots is exemplary.