Locke, John (1632–1704)

DOI: 10.4324/9780415249126-DA054-2
Version: v2,  Published online: 2017
Retrieved December 18, 2018, from

5. Knowledge and judgement

According to Locke, the objects of our knowledge are propositions. Verbal propositions are constituted by verbal signs, such as the words ‘swan’ and ‘white’. Each referring word in a verbal proposition signifies an idea that is a constituent of a mental proposition. These ideas, in turn, signify things in the world or in the mind. Both verbal propositions and mental propositions can be true or false. Furthermore, there is a derivative sense in which the ideas themselves can be said to be true or false in so far as they signify things in the world. For Locke, the fundamental act of ratiocination is the perceiving of the agreement or disagreement of two ideas. This is what constitutes knowledge in its most basic sense: to know is to perceive the agreement or disagreement of ideas.

There are four sorts of agreement or disagreement: identity or diversity, relation, co-existence or necessary connection, and real existence. From these resources Locke derives three degrees of knowledge: intuitive, demonstrative and sensitive knowledge. The first and most certain degree is intuitive knowledge which is the immediate perception of the agreement or disagreement without any effort on the part of the understanding or any assistance from other ideas. ‘Black is not white’ is his stock example. The sort of disagreement involved here is diversity. Demonstrative knowledge is less certain than intuitive knowledge because it requires the assistance of intermediate ideas. Nonetheless, demonstrative knowledge is fully-fledged knowledge because each step in its acquisition involves intuitive knowledge. Locke’s clearest example of this form of knowledge is the perception of the equivalence of ideas of the length of the sides of an equilateral triangle with the assistance of ideas of two circles placed so that the base of the triangle is a radius of each circle (Locke 1823, vol. 4: 232). In this example, the intermediate ideas are the circles and the sort of agreement is relation, the relation of equivalence.

Locke is not as clear as we would like concerning the precise relation between the act of perceiving the agreement of two ideas on the one hand and the formation of a mental proposition expressing that agreement on the other. Many scholars claim that, for Locke, to perceive the agreement of two ideas just is to form the mental proposition. It may be, however, that Locke’s view is that perceiving the agreement of two ideas is a necessary but temporally prior act to forming the mental proposition in the same way that forming the verbal proposition requires the prior formation of the mental proposition.

Another point at which interpreters would have liked greater clarity on Locke’s part concerns his comments on the third degree of knowledge: sensitive knowledge. This is knowledge of the existence of particular things in the external world. At first blush it might seem that sensitive knowledge does not satisfy Locke’s definition of knowledge as the perception of the agreement or disagreement of ideas because it involves a connection between ideas and things in the world. A variety of suggestions have been offered to resolve this problem. It has been argued, for example, that sensitive knowledge is not really genuine knowledge after all. Again, it has been argued that the perception of agreement in this case is a kind of dual-aspect relation relating both ideas to each other and ideas to things in the world. Perhaps the most natural interpretation, however, is that based on a passage in Locke’s Second Reply to Stillingfleet where he claims that the two ideas perceived to agree in sensitive knowledge are ‘the idea of actual sensation ... and the idea of actual existence of something without me that causes that sensation’ (Locke 1823, vol. 4: 360). The idea of actual sensation will be, say, the simple idea of a sphere caused by the ball in front of me. The idea of actual existence of something without me is the idea of an externally existing spherical object.

Locke’s expression ‘idea of actual sensation’ here is equivalent to ‘actual sensory idea’. In the example of the acquisition of the idea of a sphere, the idea is tagged as deriving from sensation and not, say, from memory or reflection. Sensitive knowledge then, at least on this interpretation, is the agreement of sensory ideas, that is, ideas acquired through the senses, with the idea of real existence extrinsic to the mind. Sensitive knowledge implies a representative theory, rather than a direct realist theory, of sensory perception (see Perception). If this interpretation is correct, then the idea of real existence features in each of Locke’s three degrees of knowledge: we have intuitive knowledge of our own existence; demonstrative knowledge of God’s existence – Locke offers a somewhat convoluted version of the cosmological argument (Essay IV.x); and sensitive knowledge of the existence of the external objects that we perceive via the senses.

According to Locke, our knowledge is subject to severe constraints. Not only do we lack knowledge of anything beyond our ideas, we even lack knowledge in those cases where we fail to perceive the agreement or disagreement of those ideas that we do have. In these cases, however, we often suppose that there is an agreement or we perceive a probable relation between the ideas. Locke calls this judgement, and judgement makes up for some of what we lack in terms of knowledge. Unlike knowledge, which is absolutely certain, judgement can vary in its degree of certainty. Or, to put it another way, we can hold different degrees of assent towards propositions that are probable.

There are two sorts of such propositions about which we judge: first those concerning matters of fact that we observe or that we learn from the testimony of others, and second matters of fact that lie beyond the reach of the senses. Locke has influential views on each of these sorts of probable proposition and the means by which we come to apportion degrees of assent to them. His treatment of the first sort – matters of fact to which we or others have epistemic access – leads him to a survey of the nature and role of testimony. This includes a consideration of the testimony of one who cannot lie, namely, God, which in turn leads him to set out the distinction between faith and reason and the antithesis of reason: enthusiasm. His discussion of the second sort of propositions about which we judge – matters of fact that are beyond the limits of our senses – leads him to treat of the method of natural philosophy. Let us examine each of these issues in turn.

Citing this article:
Anstey, Peter. Knowledge and judgement. Locke, John (1632–1704), 2017, doi:10.4324/9780415249126-DA054-2. Routledge Encyclopedia of Philosophy, Taylor and Francis,
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