The Genealogy of Morality (1887) is Nietzsche's most sustained and carefully argued work, and its first treatise contains an argument that, once you have understood it, is very hard to un-see. The argument is a historical-psychological genealogy: not a logical derivation of where our moral values come from, but a naturalistic investigation of the actual psychological and social conditions that produced them.
Nietzsche begins by identifying two fundamentally different patterns of moral evaluation in human history. The first, which he calls master morality, begins with a positive evaluation: the nobles or masters look at themselves, their strength, their vitality, their creative power, and call it good. Whatever is weak, sickly, or reactive is then called bad, not evil, just bad, lesser, to be pitied or dismissed. The evaluative center of gravity is the masters themselves. Their morality is affirmative, self-referential, and aristocratic.
The second pattern, slave morality, begins with a negative evaluation: the slaves look at their oppressors, their power and vitality, and call it evil. Then, by negation, they define themselves as good, as the opposite of the evil masters. The evaluative center of gravity is the enemy. Slave morality is reactive, other-referential, and born of what Nietzsche calls ressentiment: a French word for resentment, but carrying a specific philosophical weight. Ressentiment is not just ordinary resentment. It is the accumulated, suppressed, powerless rage of those who cannot respond to injuries with action, and who therefore respond with imagination, with a fantasy of moral inversion in which the powerful are evil and the weak are holy.
Nietzsche's most controversial historical claim: the triumph of slave morality in the West is the triumph of what he calls the slave revolt in morality, executed historically by the Jewish priestly class and completed by Christianity. The priests, who had no physical power over the nobles, revalued the entire field: poverty became holiness, weakness became humility, powerlessness became meekness, suffering became virtue. The meek will inherit the earth. The first will be last. The values of the masters, strength, vitality, pride, dominance, were recast as sin, arrogance, worldliness, and wickedness. And this revaluation, astonishingly, succeeded: it is this value system, Nietzsche argues, that governs modern Europe in secular dress, even among people who have abandoned Christianity.
To understand why Nietzsche thinks this is a disaster, you need his concept of will to power. Despite how it sounds, this is not primarily about political domination or conquest. It is Nietzsche's term for the drive toward self-overcoming, toward growth, toward the expansion of one's capacities and the expression of one's particular character. What slave morality does, he argues, is precisely to suppress this drive: it teaches people that suffering is punishment, that pride is sin, that self-assertion is wickedness, that the excellent are no better than the mediocre. A culture saturated with slave morality is a culture in which the conditions for genuine human excellence are systematically undermined, not by brute oppression but by the internalization of values that make excellence seem shameful.
A critical note on what Nietzsche is and is not saying: the SEP is careful to point out that his objection to MPS is about its effects on a moral culture, not about whether any particular philosophical theory explicitly requires excellent persons to suffer. When the norms against suffering and for happiness dominate a culture, potentially great people internalize them and direct their energies toward comfort and self-pity rather than toward the difficult, demanding work of genuine creation. His critique is empirical, not purely theoretical: this is what he thinks actually happens when a culture is governed by the values of compassion and equality as supreme virtues.
And now comes the question that makes the Genealogy so uncomfortable: how much of your own moral psychology is slave morality in this sense? When you feel moral indignation at the successful, the confident, the powerful, is that a genuine ethical response, or is it the ressentiment of someone who would rather condemn excellence than pursue it? Nietzsche does not give you an easy out here. He is happy to make you sit with the question.