In Natural Right and History, Strauss insists that Locke's political philosophy is a revolutionary break with the philosophic tradition of classic natural right.
Through the shift of emphasis from natural duties or obligations to natural rights, the individual, the ego, had become the center and origin of the moral world, since man--as distinguished from man's end--had become that center or origin. . . . The world in which human creativity seems to reign supreme is, in fact, the world which has replaced the rule of nature by the rule of convention. From now on, nature furnishes only the worthless materials as in themselves; the forms are supplied by man, by man's free creation. For there are no natural forms, no intelligible "essences": "the abstract ideas" are "the inventions and creatures of the understanding, made by it for its own use." Understanding and science stand in the same relation to "the given" in which human labor, called forth to its supreme effort by money, stands to the raw materials. There are, therefore, no natural principles of understanding: all knowledge is acquired; all knowledge depends on labor and is labor. (pp. 248-49)
Against this claim that Locke denies "the rule of nature," we might notice Locke's constant appeals to "the principles of human nature," which include the natural desires for survival, reproduction, social life, and knowledge, as expressing the natural pursuit of happiness as the ultimate end of human action (FT, 88-97; ST, 10, 67). "The highest perfection of intellectual nature lies in a careful and constant pursuit of true and solid happiness" (ECHU, 2.21.52).
In recognizing this Lockean appeal to human nature and the nature of human happiness, I agree with Tom West and with Peter Myers in his book Our Only Star and Compass: Locke and the Struggle for Political Rationality (Rowman and Littlefield, 1998).
Strauss assumes that in criticizing reasoning about "essences," Locke is rejecting the reality of "natural forms" altogether. But Strauss fails to see how this Lockean skepticism about Platonic essentialism is grounded on an empirical science of biological natural history that goes back to Aristotle, which was to be fulfilled by Darwinian evolutionary science.
In the Essay Concerning Human Understanding, Locke rejects those scholastic proponents of reasoning about eternally fixed essences, because they "suppose their words to stand also for the reality of things," and because they understand natural science to consist in "the bare contemplation of . . . abstract Ideas" (3.2.5; 4.12.9). By contrast, Locke argues, "to define names right, natural history is to be enquired into; and their properties are, with care and examination, to be found out" (3.11.24). Rather than studying human understanding through the logical analysis of mental abstractions, Locke relies on "this historical, plain method" of reasoning from observational experience of what actually happens (Intro., 2).
Rather than deductive reasoning from supposedly eternal and fixed "essences," Locke thus appeals to natural history as the way to understand "the nature of things themselves" through experience and observation. Such knowledge does not permit demonstration and certainty, but it does provide probabilistic knowledge. We can thus know "the regular proceedings of causes and effects in the ordinary course of nature," and this we call "an argument from the nature of things themselves" (4.16.6). Nature is so highly variable and contingent that we cannot reliably discover the unchanging essences sought by the scholastics. But we can discover with some probability the regular patterns in the natural world. "I would not here be thought to forget, much less to deny, that nature in the production of things, makes several of them alike: there is nothing more obvious, especially in the races of animals and all things propagated by seed" (3.3.13).
This reference to the natural order of the living world of plants and animals is significant because it manifests Locke's biological understanding. Locke was a medical doctor and medical researcher. In his personal library, he had more medical books than books of philosophy. He was an associate of Thomas Sydenham, the most prominent medical doctor and researcher of his time, who promoted medicine as an empirical science of natural history. Similarly, Locke's philosophical science of human nature was an empirical, probabilistic science that looked for recurrent patterns in the variable phenomena of human thought and action, a science of life that could be traced back to Aristotle and that was carried forward by Darwin.
That Locke is drawing from the Aristotelian biological tradition of understanding species is evident when one notices the passages in the Essay that are almost direct quotations from Aristotle's biological writings. For example, Locke's comments on how "we shall find everywhere that the several species are linked together, and differ but in almost insensible degrees" (3.6.12; 4.16.2) echo passages in Aristotle's Part of Animals (681a10-15). Like Locke, Aristotle is criticizing the Platonic tradition of essentialism in defending a biological concept of species rooted in an empirical science of natural history.
Some related posts can be found here, here, here, here, here, here, and here.