Natural Right and History, University of Chicago Press, 1953.  Reprinted: University of Chicago Press, 1965.

Excerpt:

It would seem, then, that the rejection of natural right is bound to lead to disastrous consequences. And it is obvious that consequences which are regarded as disastrous by many men and even by some of the most vocal opponents of natural right do follow from the contemporary rejection of natural right. Our social science may make us very wise or clever as regards the means for any objectives we might choose. It admits being unable to help us in discriminating between legitimate and illegitimate, between just and unjust, objectives. Such a science is instrumental and nothing but instrumental: it is born to be the handmaid of any powers or any interests that be. What Machiavelli did apparently, our social science would actually do if it did not prefer—only God knows why— generous liberalism to consistency: namely, to give advice with equal competence and alacrity to tyrants as well as to free peoples.2 According to our social science, we can be or become wise in all matters of secondary importance, but we have to be resigned to utter ignorance in the most important respect: we cannot have any knowledge regarding the ultimate principles of our choices, i.e . , regarding their soundness or unsoundness; our ultimate principles have no other support than our arbitrary and hence blind preferences. We are then in the position of beings who are sane and sober when engaged in trivial business and who gamble like madmen when confronted with serious issues—retail sanity and wholesale madness.

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