For Nietzsche decandence is the symptom which led him to discover the modern illness - nihilism. It is a Darwinian genealogical disease - not necessarily one that creates inferior genes (though with the rise of epigenetics it is easy to speculate that this may not be the case), but rather one which creates inferior beings.
- Some humans are naturally superior to others
- Every superior type of man represents a type in the direction in which human beings ought to will their own development.
Nietzsche imagines original human societies as ones with very clear and healthy sets of values. Since all societies, for Nietzsche, originated in an act of domestication of unshaped roving populations by domineering individuals it was natural that such domineering individuals would create societies around those values they enjoyed. What were these values? Beauty, Intelligence, Health, Power, Vitality, Cheerfulness, etc. These were the values of the pre-Socratic Greeks. They represent a desire by humanity and human instincts to amplify all the qualities which man finds good in himself. These are the original natural values - valued in all animals. However, unlike other animals, mankind has the option to will these values or not to will these values. In addition, mans peculiar potential to shape his own future, allows him to will higher men embodying those things which he values but does not himself fully embody. The "good" - in short, those embodying these values and with power in a society, the masters - set up and opposed themselves to the "bad" - in short, those not embodying these values and without power in society. These were not, originally, character judgmenets or moral judgmenets, but instead merely names for attributes and social positions. The "good" embodied the master's values, the "bad" did not.
This is the original "pathos of distance" out of which arose masters and slaves. The slaves, for a time, accepted their lot and even the values of their masters. Slaves have always accepted their masters values, for on "instinct" alone they are clearly superior. It was here, however, that resentment first took hold of those few lowly slaves who coveted their masters power and hated themselves. These botched artists here sought to make themselves masters without embodying any masterly qualities. This, however, required a ground prepared for such a revaluation. In these men alone the soil was already ripe - for they were resentful and hateful enough to believe their own words - but the rest of the population required preparation and an induction into these values. These first few resentful slaves sought to kick their masters from their thrones. Under the masters values this would be impossible. These slaves were neither beautiful nor intelligent. Neither healthy nor powerful. What they needed was a new, counterfeit set of values. Ones that would allow themselves to become that which humanity desired. If they were to be seen as such, however, some powerful new tools were needed. Since resentment turns to hatred of the master what was needed was a new master. This new master must champion the slaves and condemn the masters. This new master must be above all possible human masters so that his word is absolute. So that he can never be supplanted by anything human. In addition, he must be singular, so that no in-fighting among masters can ever occur. So that all slaves can be united under a single Father-like figure. Enter God - the singular, unambiguous, supreme master over all creation. God was the original slave invention, the first tool with which the slave revolt in values would begin.
Given such a new master - one which was clearly for the slaves and against the current masters - what sorts of values would he champion? What sorts of values would he condemn? Since he is the champion of the ugly, the weak, the sick, the stupid, and the hateful he must of course despise the beautiful, the strong, the healthy, and the smart. He must choose the lowly slaves as his "chosen people" and command them against their masters - for they embody all the lowliest values. Since God is all powerful he smites those who think themselves high, find themselves high in society. He champions those that are low, those that embody nothing, stand for nothing, can produce nothing, and value nothing. Since they are the sick he must prescribe to them some health measures, but these must not be so extreme as to make them well. For the sickness of the sick must be preserved if one is to keep ones flock faithful and desperate for cures.
Christianity, however, is the first to invent a whole new set of tools. God of the old testament was merely hatred to the masters through and through. With Christiantiy the weak come to overthrow the strong - to make even the strong hate themselves. For this, a new and unprecedented tool was need. A psychological device which would cause the masters to hate themselves, glorify the weak, and become weak themselves. For this, a variety of whips are needed. The strong must weaken themselves whenever they recognize their own strength. Become disgusted at any display of superiority, and so learn to automatically lower themselves should they ever feel raised. For this were devised several instruments - guilt, pity, humility, sin, repentence. A repertoire of psychological poisons which, when combined, could bring even a Titan to his knees. However, the poor, being sickly themselves, felt drawn to these tools on instinct alone. These were their natural tools. With the widespread deployment of these tools begins the era of decadence. The strong are now raised to be lowered. The weak, raised above the tortured strong for such a corruption and sickness is, to them, no torture at all but merely their baseline.
Guilt - In short, self-torture over past miscalculations beyond what is reasonable. Before, one would merely think "I have miscalculated." or "That was stupid." but never "I am evil". Pity - In short, the will to preserve all that is despicable and broken. Humility - In short, the will to reduce all distances between man and man. Sin - In short, the will to condemn oneself for ones own misfortunes. Repetence - In short, the will to apologize for sin to expiate it (Requires a priest to repent to. This is important, as it sets the priest up as the only one capable of expiating the psychological torture wrought by these tools. Thus, the priest is raised above all men as the only one knowing how to heal the wounded).
Nietzsche names this triumph over the masterly values the "slave revolt in morals". The human species had found a way to will the opposite of all that was pleasing and highest in itself in favor of a leveled society in which none triumphed over others. He saw this as the triumph of mediocrity. Ideas at once compelling and extremely controversial well over 100 years later.