I have described the spectrum of political ideologies as a circle. But as I rethink my analysis, I conclude that the spectrum should be thought of a straight line. At the left is statism (which is really leftist even when it is said to be rightist), conservatism is in the middle, and pure libertarianism (anarchy) is at the right.
Statism is statism: The ruler or ruling class decides how you should live and ensures, through physical and psychological coercion that you live as you are told to live.
Anarchy is the opposite of statism: No one is in charge of everyone. Whether anarchy is good or bad depends on the morals of the potentially most powerful persons or coalitions.
Conservatism is therefore centrist because it recognizes the need for a state of defined and limited power -- just enough power to enforce a benign morality.