AMIDST the desert of American left-wing thought over the past decade, the writings of Michael Walzer have appeared as oases of intelligence, grace, and sobriety. Walzer’s political essays in Dissent, The New Republic, and the New York Review of Books, many of which have now been collected and reprinted under the title Radical Principles, are written in dear, forceful, and sometimes elegant prose. Now at Princeton’s Institute for Advanced Studies, after a lengthy tenure as a professor of government at Harvard, he is learned in-and his thought to some extent has been shaped by-the great works of the Western philosophic tradition. Karl Marx is the thinker he most often cites, but his arguments are studded also with references to Hobbes and Locke, to Machiavelli and Rousseau.  Most of the leading left-wing authors of the past century and a half are scarcely mentioned at all. The tone of Walzer’s writing is generally moderate and judicious (though his remarks on the Vietnam War are an exception to this rule); his unmistakable moral seriousness rarely degenerates into the moral stridency that so often characterizes the American Left.