Some guy named Aristotle said the following, which can be found in book 8, chapter 10 of Nicomachean Ethics:
Now there are three forms of constitution, and also an equal number of perversions or corruptions of those forms. The constitutions are Kingship, Aristocracy, and thirdly, a constitution based on a property classification, which it seems appropriate to describe as timocratic, although most people are accustomed to speak of it merely as a constitutional government or Republic.
The best of these constitutions is Kingship, and the worst Timocracy. The perversion of Kingship is Tyranny. Both are monarchies, but there is a very wide difference between them: a tyrant studies his own advantage, a king that of his subjects. For a monarch is not a king if he does not possess independent resources, and is not better supplied with goods of every kind than his subjects; but a ruler so situated lacks nothing, and therefore will not study his own interests but those of his subjects. A king who is not independent of his subjects will be merely a sort of titular king. Tyranny is the exact opposite in this respect, for the tyrant pursues his own good. The inferiority of Tyranny among the perversions is more evident than that of Timocracy among the constitutions, for the opposite of the best must be the worst.
When a change of constitution takes place, Kingship passes into Tyranny, because Tyranny is the bad form of monarchy, so that a bad king becomes a tyrant. Aristocracy passes into Oligarchy owing to badness in the rulers, who do not distribute what the State has to offer according to desert, but give all or most of its benefits to themselves, and always assign the offices to the same persons, because they set supreme value upon riches; thus power is in the hands of a few bad men, instead of being in the hands of the best men. Timocracy passes into Democracy, there being an affinity between them, inasmuch as the ideal of Timocracy also is government by the mass of the citizens, and within the property qualification all are equal. Democracy is the least bad of the perversions, for it is only a very small deviation from the constitutional form of government. These are the commonest ways in which revolutions occur in states, since they involve the smallest change, and come about most easily.