Robin's law is named for Broolyn College political science professor Corey Robin. Not because he proposed it (I'm proposing it here, and haven't seen it formulated elsewhere, though I suspect it's not original), but because he is a fine example.
The law is thus:
Ideologues frequently view their political opponents as little more than inversions of themselves.Robin's entertaining anti-conservative polemic The Reactionary Mind, which has made many liberals stand up and cheer (and contains many good points within its pages), is a textbook example. The thesis of Dr. Robin's work is that the main motivation of conservatism, as manifested throughout the ages, is maintaining an aristocracy (of some sort or another) against the interests of the broader populace.
Many other liberals (including myself, in my less reflective moments) make the same category error. Liberals are often motivated by issues such as economic equality and social justice, and thus frequently conclude that conservatives are motivated by inequality and injustice--that these things are the raison d'etre of conservatism. While there are doubtless many powerful rich folk in the conservative movement who's goal is to undermine the working class--and will marshal any number of other arguments to support this cause--that alone cannot explain the bulk of conservative politics. (Some liberals act as though the broader conservative movement has been brainwashed in some fashion by the plutocracy--an allegation which is insulting nonsense).
Conservatives, of course, commit the same error in their views of liberals, in spades. Conservative discourse is full of portrayals of liberals as lazy, nihilistic, hedonists. On bad days, liberals are often described as agents of either foreign states or of the Devil. Many conservative pundits argue with a straight face that the Democratic Party--an institution which 31% of Americans identity with--is an organization of traitors. It is assumed by many on the right that the purpose and fundamental goals of Democratic policy is such things as "undermining family values" or weakening US security. Which is, of course, news to any card-carrying Democrat.
Libertarians, though, may be the champions of this. Libertarians have seemingly fabricated an entire ideology--"statism"--which they ascribe to their opponents, left and right. In this reckoning, the goal of the statist is--as the name suggests--is to expand the scope of the state. Rather than the state being an instrument towards some other policy end (such as restricting access to narcotics or mitigating the effects of poverty), the cart is placed before the horse, and these policy initiatives are merely fig-leafs to justify the true end goal--growth of the Leviathan.
In all three cases, these ridiculous (but popular) caricatures have the same root cause: Rather than attempting to honestly understand the motivations of one's opponents, ideologues will assume that the opponents are simply their opposites--opposed to what they consider to be good, and supportive of what they consider to be evil. Liberals, interested in economic justice, accuse conservatives of championing plutocracy. Conservatives, interested in a strong moral order and a strong national defense, treat liberals as though they wish to destroy morality and surrender to foreign enemies, real or imagined. And libertarians, desiring to shrink the state out of first principles, assume their adversaries are motivated by a desire to enlarge it.