It seems Leo Strauss is referring to one of two people - both of whom he greatly admired: either Heidegger or Schmitt. Which is the more likely candidate?
It is only at this point that we come face to face with the serious antagonist of political philosophy: historicism. After reached its full growth historicism is distinguished from positivism by the following characteristics. (1) It abandons the distinction between facts and values, because every understanding, however theoretical, implies specific evaluations. (2) It denies the authoritative character of modern science, which appears as only one form among many of man's thinking orientation in the world. (3) It refuses to regard the historical process as fundamentally progressive, or, more generally stated, as reasonable. (4) It denies the relevance of the evolutionist thesis by contending that the evolution of man out of non-man cannot make intelligible man's humanity. Historicism rejects the question of the good society, that is to say, of the good society, because of the essentially historical character of society and of human thought: there is no essential necessity for raising the question of the good society; this question is not in principle coeval with man; its very possibility is the outcome of a mysterious dispensation of fate. The crucial issue concerns the status of those permanent characteristics of humanity, such as the distinction between the noble and the base, which are admitted by the thoughtful historicists: can these permanencies be used as criteria for distinguishing between the good and bad dispensations of fate? The historicist answers this question in the negative. He looks down on the permanencies in question because of their objective, common, superficial and rudimentary character: to become relevant, they would have to be completed, and their completion is no longer common but historical. It was the contempt for these permanencies which permitted the most radical historicist in 1933 to submit to, or rather to welcome, as a dispensation of fate, the verdict of the least wise and least moderate part of his nation while at the same time to speak of wisdom and moderation. The biggest event of 1933 would rather seem to have proved, if such proof was necessary, that man cannot abandon the question of the good society, and that he cannot free himself from the responsibility for answering it by deferring to History or to any other power different from his own reason. ("What is Political Philosophy?" in What is Political Philosophy? And Other Studies, 26-7)
As a matter of context, "What is Political Philosophy?," is the published version of the Judah L. Magnes lectures Strauss delivered at the Hebrew University in December 1954 and January 1955.
(Cross-posted to theoria.)