One of the most basic themes of Locke's epistemology is that since we cannot know everything, we would be well-advised to observe and respect the extent and limitations of human knowledge. Given the basic definition of knowledge as perception of the agreement of our ideas, it follows that we fall short of knowing whenever we lack ideas or fail to perceive their agreement. Thus, intuition extendes only to the identiy and diversity of ideas we already have; demonstration extends only to ideas between which we are able to discover intermediaries; and sensitive knowledge informs us only of the present existence of causes for our sensory ideas. [Essay IV iii 3-5] Awareness of our limitations, Locke proposed, should forestall haste, laziness, and despair in our natural search for the truth about the most vital issues into which human knowers can fruitfully inquire. [Conduct 39-43]
Applying the human faculty of reason to the pursuit of knowledge, properly defined, reveals the limitiations within which we must work: We cannot achieve knowledge of thingssuch as infinity or substantial real essencesfor which we lack clear, positive ideas. Indeed, having ideas will not be enough to secure knowledge ifas in the case of human actionsthey are obscure, confused, or imperfect. Given faulty memories, we may also fail to achieve knowledge because we are incapable of tracing long chains of reasoning through which two ideas might be demonstrably linked. In a more practical vein, rational knowledge cannot be established upon false principlessuch as those borrowed from conventional wisdom. Finallyin the effort to achieve philosophical or scientific certaintyour efforts to employ reason are commonly undermined by the misuse or abuse of language. [Essay IV xvii 9-13]
Most particularly, on Locke's view, it is difficult to secure the reality of human knowledge in any evidence of its conformity with the nature of things themselves. We readily assume that passively-received simple ideas must be providentially connected with their objects, and since complex abstract ideas are of our own manufacture, it is our own responsibility to ensure their reality by a consistent use of the names by which we signify their archetypes. But complex ideas of natural substances are intended to represent the way existing things are independently of our perception of them, and of this the content of our ideas never provides adequate evidence. [Essay IV iv] These difficulties trouble all four types of knowledge.
Since knowledge of identity and diversity involves only a recognition that particular ideas are distinct, it is immediately evident whenever we have clear and distinct ideas; such knowledge must be among the earliest that any of us ever achieve. [Essay IV vii 9-11] Even otherwise ignorant beings might well be capable of perceiving the disagreement of ideas at this level, Locke pointed out, so knowledge of "identical Propositions" of this sort is generally uninformative (or "trifling") rather than any genuinely instructive contribution to morality or science. [Essay IV viii 2-3] The general principle of identity merits no special status among our cognitive states, Locke held, since its particular instances are all either equally obvious or (because of the obscurity of our ideas) irredemable without it. [Essay IV xvii 14-19]
Knowledge of relation requires only that we are aware of non-identical connections among ideas, so (as we'll see in greater detail later) Locke supposed it possible wherever we have clear ideas, especially among the simple modes of number in mathematics and the mixed modes of human action in morality. [Essay IV vii 6] But since general knowledge of relations can never be derived from experimental observationupon which we depend entirely for our knowledge of material thingsit follows that we can never have certain knowledge of relations among substances. [Essay IV vi 10, 16]
What careful observation does provide is knowledge of the co-existence of a collection of qualities in a common subject. But because we are ignorant of the real essences from which observable qualities presumably flow, our knowledge of co-existence is never demonstrable, except in the trifling cases where we have already included the ideas of the qualities in our nominal essence for substances of this sort. On Locke's view, then, natural science founded firmly upon solid understanding of the inner constitution and operation of material things remains impossible. [Essay IV vi 6-10]
Even knowledge of real existence requires some awareness of the connection between the thing and the idea that represents it, and Locke supposed that we lack this in every case except that of self and God. [Essay IV vii 7] The extent of our perfect knowledge of the world and its operations is meager indeed.
But perhaps we don't need perfect knowledge very often. Although Locke emphasized the strict limits within which we can attain certain knowledge, he also believed that we invariably possess the cognitive capacities that will provide for the conduct of our everyday lives.
Although we'd soon starve if we waited for demonstrative certainty about the nutritional value of food, none of us forget to eat, since human life in what Locke called the "twilight . . . of Probability" is providentially supplied with alternative methods and motives for practical action. Rational proof is impossible for experimental natural science and uncommon in other matters regarding which we are too lazy to work through the demonstrations on our own, Locke supposed, but in these cases we don't really have to have genuine knowledge. It is often enough to exercise the faculty of judgment, which accepts a presumptive agreement between ideas without demanding the certainty of a clear perception. [Essay IV xiv 3-4]
The Understanding Faculties being given to Man, not barely for Speculation, but also for the Conduct of his Life, Man would be at a great loss, if he had nothing to direct him, but what has the Certainty of true Knowledge. For that being very short and scanty, as we have seen, he would be often utterly in the dark, and in most of the Actions of his Life, perfectly at a stand, had he nothing to guide him in the absence of clear and certain Knowledge. [Essay IV xiv 1]
Functioning properly, this faculty persuades us to assent to propositions about whose truth we remain ultimately uncertain, whether our ignorance is the product of incomplete thinking or the secret nature of the thing itself. [Essay IV xvii 22] Although mathematicians can demonstrate that the interior angles of every plane triangle sum up to 180 degrees, for example, most of us rely upon their professional testimony rather than following the train of reasoning for ourselves. Similar degrees of reliance upon less-than-demonstrative certainty Locke believed to support most of the propositions to which we commonly grant our assent. [Essay IV xv 1-2] When we lack demonstrative certainty, the role of judgment is to guide our actions in all of those cases regarding which the best we can do (or the best we are willing to do) is to observe that a pair of ideas often do seem to be related to each other in some way. [Essay IV xvii 16]
The probable knowledge gained by judgment differs from the demonstrative knowledge derived by reason in the nature of its evidence: while demonstration achieves perfect certainty by grounding itself on the clear, immutable connection of two ideas, probable knowledge merely guides our judgment with some degree of likelihood or the appearance of some connection. Thus, while demonstration yields true knowledge, judgment can provide only some degree of confidence in our opinion, assent, or belief. [Essay IV xv 1-3] The great sources of evidence for our confident assurance of the likelihood of propositions to which we assent by judgment are our own experience and the testimony of others about what they have experienced. Noting that we commonly evaluate the reliability of testimony by reference to our own experiences anyway, Locke proposed that we can and should depend more often upon things we have personally observed with some regularity than upon what could turn out to be nothing more than the prejudices or false opinions of other people. [Essay IV xv 4-6]
Although the degree to which we assent to a probable proposition ought to depend upon the strength of the evidence in its favor, Locke granted that we often regulate our judgment merely be reference to our faulty memories of past experience. It is both natural and (in a practical snese) necessary to rely upon the retention of past experience rather that developing our beliefs anew in every moment, it is a dangerous practice, because the propositions we acquire through exercise of the faculty of judgment are bound to remain genuinely uncertain. Since the supposed relation between the ideas is founded only upon our present estimation of the available evidence, it is always possible in principle that the discovery of additional information in the future may lead us to overthrow or abandon a past judgment, as can never happen with truly demonstrative knowledge. The difficulty of applying judgment successfully, Locke suggested, should encourage us to be patient and tolerant of those who disagree with us on matters about which neither side can claim anything more sure than probable opinion. [Essay IV xvi 1-4]
The highest possible degree of probable knowledge will occur in cases where the general consent of all human beings happens to coincide with my own invariable experience of some particular matter of fact. This, Locke supposed, will produce a level of assurance virtually indistinguishible from that of demonstrative certainty, and this explains our willingness to act without hesitation upon our conviction that such beliefs truly capture the nature of reality, even though we remain ignorant of the inner constitution of things themselves. In cases that exhibit a less striking regularity in my own experience and that of others, the degree of my confidence in the probable proposition will be suitable reduced, Locke held, and even in the absence of any direct observation regularity, I am likely to accept the unanimous testimony of impartial witnesses with respect to any specific matter of fact. [Essay IV xvi 6-8] When experience and testimony do not so clearly agree, Locke supposed, other methods may serve to guide the dictates of judgment. In legal and quasi-legal contexts, we develop a great deal of skill in evaluating the relative merits of conflicting testimony from distinct sources. In the natural sciences, we commonly employ analogical models in an effort to comprehend the real essences of which we are constitutionally ignorant. [Essay IV xvi 10-12]
In addition to all of these legitimate grounds for guidance, the faculty of judgment commonly falls victim to unworthy and unsupported claims to its assent. Someone may demand that I assent to the truth of a proposition only because it is defended by some putative authority, in the absence of any proof of its falsity, or solely because it agrees with other opinions I already hold. But since all of these matters are formally irrelevant to the truth of the proposition in question, Locke supposed, they should have no bearing on my assent. The only legitimate grounds for agreeing with someone are demonstrative knowledge and probable judgment, both of which ultimately rest only upon "the nature of Things themselves."
[Essay IV xvii 19-22]