John Locke's empirical project is founded upon the axiom that the components of thought are the product of experience, and that knowledge is thereby the result of reflection and sensation. For Locke, knowledge is "the perception of the connexion and agreement, or disagreement ... of any of our ideas." Towards the end of unpacking such connections, Locke outlines "three degrees of knowledge, namely, intuitive, demonstrative, and sensitive, in each of which there are different degrees and ways of evidence and certainty." Intuitive and demonstrative knowledge he defines as the immediate and intermediate connection of ideas respectively, and thus their correspondence with his definition of knowledge is readily apparent. With respect to sensitive knowledge however, such a correspondence is not so easily found. Sensitive knowledge he describes as a "perception ... employed about the particular existence of finite beings without us." Here a potential problem arises; by Locke's definition knowledge does not result from the connection of an internal idea and an external object, and if knowledge is the perception of psychical ideation, then how is the physical sensation of "beings without us" involved in such a perception? Because of this potential complication, some observers have claimed "Locke doesn't ... mean to define all knowledge in terms of" ideas, or else "that he errs in extending knowledge to external existence." Here we will examine whether defining knowledge as a connection of ideas precludes the possibility of having sensitive knowledge by sensations of the external world, and argue that Locke's definitions of knowledge and sensitive knowledge do in fact cohere, and hence that he did not err in extending knowledge to include external reality.

To understand the problem of designating sensitive knowledge as knowledge using Locke's definition, we must first understand the differentiations in his "three degrees of knowledge," in particular with respect to ideation, certainty, and error.

The first degree of knowledge, intuitive, is the "immediate comparison" of ideas, and possesses a certainty "wherein there is no room for ... doubt." Here we see the direct correspondence of intuition with Locke's definition of knowledge by ideas, and no further discussion is necessary.

The second degree of knowledge, demonstrative, is the perception of the connection between ideas that arises from their reasoned combination across multiple stages, through the usage of "intermediate ideas" as perceptual aids. The use of intermediate ideas requires the use of memory, and consequently there exists room for perceptual error, albeit as a mere "lessening of that perfect clearness and distinctness which is" found in intuitive knowledge, and being therefore in no way an abrogation of the ability to perceive ideational connections. Here again we find a direct correspondence to Locke's definition of knowledge by ideas.

The third degree of knowledge, sensitive, is "another perception of the mind, employed about the particular existence of finite beings without us, which, going beyond bare probability, and yet not reaching perfectly to either of the foregoing degrees of certainty, passes under the name of knowledge," and provides us "with an evidence that puts us past doubting." In this case there is no obvious correspondence between Locke's definitions of knowledge and sensitive knowledge. In Locke's explication of sensitive knowledge as quoted above, Locke claims sensitive knowledge does indeed pass "under the name of knowledge," while at the same time not "reaching ... the foregoing degrees of certainty" captured by intuitive and demonstrative knowledge. The failure of sensitive knowledge to attain the certainty of intuition and demonstration obtains because, in the words of Lex Newman, it "incorporates the vulnerabilities of probable judgment" by its "relation with an external cause" -- that is, because of its relation with "finite beings without us." How then can such a relation be understood to yield knowledge via ideas? How is it that the perception of a connection between ideas may arise from, and incorporate external causes? The issue is that by Locke's own definition sensitive knowledge "passes under the name of knowledge," and therefore "probable judgment" is not what he is aiming at -- for judgment is mere presumption, rather than knowledge producing perception. The question here becomes: if sensitive knowledge "incorporates the vulnerabilities of probable judgment," in what way is it "past doubting"?

For Locke, the human epistemic system operates such that we "cannot avoid the having" of ideas about the external world that are produced by our senses. Consider for example the difference between experiencing the sun and remembering an experience of the sun. "For I ask any one, Whether he be not invincibly conscious to himself of a different perception, when he looks on the sun by day, and thinks on it by night ... ? We as plainly find the difference there is between any idea revived in our minds by our own memory, and actually coming into our minds by our senses, as we do between any two distinct ideas." Thus while we may not comprehend the underlying function of our ability to appreciate the difference between a sensitive experience (or sensitive idea) and other conditions of ideation such as remembrance, our epistemic apparatus nevertheless endows us with the ability to appreciate that distinction. We are able to "plainly find the difference" between presently experiencing the reality of the sun and contemplating an idea of it in the absence of present experience. Our ability to appreciate this difference is central to having sensitive knowledge, for as Locke proceeds to explain; sensitive knowledge arises from "the existence of particular external objects, by that perception and consciousness we have of the actual entrance of ideas from them."

Accepting our ability to appreciate the actuality of sensation and the corresponding "actual entrance of ideas," let us now examine the perception of connections of those ideas with other ideas. As noted in the introduction, sensitive knowledge must be the result of a perception of agreement of two ideas (and cannot be the result of combining an external object and an internal idea), and here in Locke's description of the "perception and consciousness we have" of "the existence of particular external objects" and the "actual entrance of ideas from them" we find the congruence between his definitions of knowledge and sensitive knowledge. The "two ideas, that in this case are perceived to agree, and do thereby produce knowledge, are the idea of actual sensation (which is an action whereof I have a clear and distinct idea) and the idea of actual existence of something without me that causes that sensation." What we have then in sensitive knowledge is the perception of a connection of an idea caused by the external world via our senses, and an idea of the actuality of sensation. The ideas perceived to agree or disagree are the sensitive idea of the object of sensation, and the idea that the sensitive idea was the product of an object of sensation. Hence, the congruence between knowledge and sensitive knowledge is the difference and agreement between having an idea and having an idea that ideas can be presently caused by an external object. "It is therefore the actual receiving of ideas from without that gives us notice of the existence of other things, and makes us know."

In this way sensitive ideas are caused by the external world, and because they are caused within us they enable our connection to it. Furthermore, because they are ideas they may interact and connect with other ideas. Thus sensitive experiences produce sensitive ideas that are tested against other ideas in the mind for agreement and disagreement, giving rise to sensitive knowledge of "particular Propositions concerning concrete Existances." This sensitive knowledge is knowledge per se by Locke's ideational definition, and it is "past doubting" because it "is the consequence of the existence of things, producing ideas in our minds by our senses." Accordingly, Locke's definition of sensitive knowledge about the external world accords with his definition of knowledge in terms of ideas.

Although the degree of certainty achieved by intuitive and demonstrative knowledge is not possible for sensitive knowledge, that fact does not preclude the achievement of sensitive knowledge as the perception of agreement of ideas. Just as we do not understand the underlying structures of our ability to produce and appreciate logical perceptions, so do we not understand the underlying framework of our ability to produce and appreciate sensitive perceptions, but in neither case does this terminate our ability to produce and appreciate those perceptions. The important distinction here then is that Locke was not mistakenly applying his ideational definition of knowledge to the perception of a connection of a psychical idea and a physical object, but was applying it to the connection of two ideas where one is sensitive and the other is the appreciation of the actuality of sensation.