And what manner of government do you term oligarchy?
A government resting on a valuation of property, in which the rich have power and the poor man is deprived of it.
And then one, seeing another grow rich, seeks to rival him, and thus the great mass of the citizens become lovers of money.
the rulers, being aware that their power rests upon their wealth, refuse to curtail by law the extravagance of the spendthrift youth because they gain by their ruin; they take interest from them and buy up their estates and thus increase their own wealth
— Plato, Republic VIII