Degrees and Types of Knowledge

Locke distinguished four types of agreement or disagreement that may be perceived in human knowledge: Knowledge of identity and diversity rests solely upon our recognition of the distinctness of each idea from every other; knowledge of relation employs positive, non-identical connections among ideas; knowledge of co-existence perceives the coincident appearance of a collection of qualities; and knowledge of real existence presumes some connection between an idea and the real thing it represents. [Essay IV i 1] He further supposed that these types of knowledge can occur in any of three degrees:

Intuitive knowledge is the irresistable and indubitable perception of the agreement of any two ideas without the mediation of any other. This is the clearest and most perfectly certain of all degrees of human knowledge. It accounts for our assent to self-evident truths and serves as the foundation up-on which all other genuine knowledge must be established. [Essay IV ii 1] Intuition is most common in our knowledge of identity and relation among clear ideas, but (following Descartes) Locke also supposed that each thinking being has an intuitive knowledge of its own existence. [Essay IV ix 3]

Even when the agreement of ideas is not intuitively obvious, it may be possible to discover a series of intermediate ideas by means reason establishes a connection between them. The resulting demonstrative knowledge of the agreement of the original ideas shares in the certainty of the intuitive steps by means of which it has been proven, yet there is some loss of assurance resulting from the length of the chain itself. [Essay IV ii 2-7]

The success of the entire process depends upon our having clear ideas at each step of the process of demonstration and upon our ability to perceive the agreements between them, and both of these conditions are specific to the nature of human intellectual abilities. [Essay IV iii 4, 26-28]

The most common area of demonstrative human knowledge is mathematics, where our possession of distinct ideas of particular quantities yields the requisite clarity, disciplined reasoning helps to uncover the intermediate links that establish knowledge of identity and relation, and a perspicuous system of symbolic representation helps us to preserve the results we have obtained. [Essay IV ii 9-10]

But Locke supposed that we may be capable of demonstrative knowledge of moral relations as well, provided that we take care in the formation of abstract ideas of the mixed modes of human action. Our only demonstrative knowledge of real existence, he supposed, is that we can have with respect to God. [Essay IV x]

Although only intuition and demonstration offer certain knowledge of general truths, Locke supposed that sensitive knowledge provides some evidence of the existence of particular objects outside ourselves. Although it is not always true that there must exist an external object corresponding to each of our ideas of sensation, Locke argued that veracious cases are different enough from illusory cases to warrant an inference to the real existence of their objects, especially when the accompanying perception of pleasure or pain serves as a reliable guide to the practical conduct of human life. [Essay IV ii 14]

Locke had serious reservations about the reliability of our sensitive knowledge of the natural world, and we'll return later to the chapter of the Essay he devoted entirely to an analysis of its difficulties. [Essay IV xi] ©1999-2002 Garth Kemerling.Last modified 27 October 2001.Questions, comments, and suggestions may be sent to:

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The Limits of Knowledge

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]

Severe Restrictions

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 things-such as infinity or substantial real essences-for which we lack clear, positive ideas. Indeed, having ideas will not be enough to secure knowledge if-as in the case of human actions-they 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 principles-such as those borrowed from conventional wisdom. Finally-in the effort to achieve philosophical or scientific certainty-our 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 observation-upon which we depend entirely for our knowledge of material things-it 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.

Assent and Judgment 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.

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] 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]

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]