Renaissance thinker Niccolò Machiavelli brought political theory into the modern age, for better or worse. In a 1971 New York Review of Books essay, Isaiah Berlin measured his impact on Western thought. An excerpt:

“If what Machiavelli believed is true, this undermines one major assumption of Western thought: namely, that somewhere in the past or the future, in this world or the next, in the church or the laboratory, in the speculations of the metaphysician or the findings of the social scientist or in the uncorrupted heart of the simple good man, there is to be found the final solution of the question of how men should live. If this is false (and if more than one equally valid answer to the question can be returned, then it is false) the idea of the sole true, objective, universal human ideal crumbles. The very search for it becomes not merely utopian in practice, but conceptually incoherent….

After Machiavelli, doubt is liable to infect all monistic constructions. The sense of certainty that there is somewhere a hidden treasure—the final solution to our ills—and that some path must lead to it (for, in principle, it must be discoverable); or else, to alter the image, the conviction that the fragments constituted by our beliefs and habits are all pieces of a jigsaw puzzle, which (since there is an a priori guarantee for this) can, in principle, be solved; so that it is only because of lack of skill or stupidity or bad fortune that we have not so far succeeded in discovering the solution whereby all interests will be brought into harmony—this fundamental belief of Western political thought has been severely shaken.” (Thanks Browser.)