Although the word "genealogy" acquired a certain cult resonance in the intellectual climate of the latter 20th century, genealogy as a so-called mode of inquiry really came of age with Nietzsche in the 19th century. Nietzsche quarreled particularly with utilitarianism, a conception of morality based on the quantification of pleasure, and shared in varying degrees by moralists such as Bentham and Mill in the English tradition.
Nietzsche's concern, however, was not morality as such, but the historical conditions in which good and bad began to be used as terms of value in the social order. In what sense, Nietzsche wondered, was morality a byproduct of the interests (or values) it served?
In "The Genealogy of Morals" (1887) Nietzsche argues that the Western notion of "good" originated with the warring nobility, an ideal which privileged strength, spontaneity, action and the immediate gratification of all feral desires. Failure to live up to this standard was simply "bad." It is important to note the amorality of this juxtaposition in Nietzsche's analysis. In the pre-moral stage of the master/slave power struggle Nietzsche envisions here, good and bad had yet to achieve meaningful opposition. In terms of social standing, therefore, slaves were merely slaves, far too "sick" or "weakly" or "lacking" to achieve goodness in the healthy "noble" sense of the word.
A split between warriors and priests soon developed within this proto-mythic ruling nobility. Lacking in physical strength (and thus in power), the priestly caste began to define itself in terms of purity, sexual and dietary abstinence, and of refraining from the carnal appetites celebrated by the warring nobility, which thrived on physical power. On account of their powerlessness, Nietzsche ultimately credits the priestly caste with one surprising feat: "one could add that man became an 'interesting animal' on the foundation of this 'essentially dangerous' form of human existence, the priest, and that the human soul became 'deep' in the higher senses and turned 'evil' for the first time..." (On the Genealogy of Morals).
Two revealing features surface in this move from "bad" to "evil." First, "evil" is an invention of the weak, of the slave or plebian who has been deprived of the power and will to act freely. It came into existence as compensation for this powerlessness, as a kind of hatred, or what Nietzsche calls "ressentiment," for all that is high-minded in the master's ideal of the good. Second, by casting the master's ideal of the good as "evil," slave morality fundamentally inverts the noble's interpretation of "good" (no longer strength but meekness, not power but humility, not acceptance of life but its negation----these will be the new moral virtues).
Nietzsche's genealogy of morals polemicizes against this reversal, against the creative cunning in slave morality that paired good and evil as natural opposites. This opposition lies at the heart of the Judeo-Christian morality like a poison, according to Nietzsche, one that has prevailed for the past 2,000 years.
Ultimately, Nietzsche's genealogy points to one overriding truth about many of the concepts we live by: their essence does not abide in timeless origins, but evolves coterminously with the historical struggle for power.