Below the rank of knowledge proper (intuitive and demonstrative), Locke recognizes a third degree of knowledge, not strictly entitled to the name. This is our sensitive apprehension of external things, or of real objects other than ourselves and God:
These two, viz. intuition and demonstration, are the degrees of our knowledge; whatever comes short of one of these, with what assurance soever embraced, is but faith or opinion, but not knowledge, at least in all general truths. There is, indeed, 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. There can be nothing more certain than that the idea we receive from an external object is in our minds: this is intuitive knowledge. But whether there be anything more than barely that idea in our minds; whether we can thence certainly infer the existence of anything without us, which corresponds to that idea, is that whereof some men think there may be a question made; because men may have such ideas in their minds, when no such thing exists, no such object affects their senses. (Bk. 4:14)
Does not the very definition of knowledge, as the perception of the agreement or disagreement of ideas with one another, preclude the perception of the agreement of ideas with non-ideal reality?
Locke's argument for the objective validity of sensitive knowledge consists of several considerations. First, he urges, our ideas of sensation differ from those of memory and imagination, that is from mere ideas, in being produced in us without any action of our own, and therefore "must necessarily be the product of things operating on the mind, in a natural way, and producing therein those perceptions which by the Wisdom and Will of our Maker they are ordained and adapted to." They,
carry with them all he conformity which is intended; or which our state requires: for they represent to us things under those appearances which they are fitted to produce in us: whereby we are enabled to distinguish the sorts of particular substances, to discern the states they are in, and so to take them for our necessities, and apply them to our uses. (Bk. 4:4:4)
Secondly, pleasure or pain often accompanies the sensation, and is absent from the idea as it recurs in memory or imagination; and "this certainty is as great as our happiness or misery, beyond which we have no concernment to know or to be" (Bk. 4:2:14). Thirdly, our several senses assist one another's testimony, and thus enable us to predict our sensational experience. On these grounds Locke concludes that,
the certainty of things existing in rerum natura when we have the testimony of our senses for it is not only as great as our frame can attain to, but as our condition needs. For, our faculties being suited no to the full extent of being, nor to a perfect, clear, comprehensive knowledge of things free from all doubt and scruple; but to the preservation of us, in whom they are; and accommodated to the use of life: they serve to our purpose well enough, if they will but give us certain notice of those things, which are convenient or inconvenient to us. (Bk. 4:2:14)
The certainty which Locke attributes to sensitive knowledge is thus seen to be practical, rather than theoretical; and it is impossible to distinguish this degree of knowledge from the belief or opinion which results from a balance of probabilities rather than from certain perception.
But even granting that our sensitive apprehensions of external reality possesses the certainty which is the characteristic of knowledge, as distinguished from mere opinion, we must observe within how very narrow limits it is confined:
When our senses do actually convey into our understandings any idea, we cannot but be satisfied that there doth something at that time really exist without us, which doth affect our senses, and by them give notice of itself to our apprehensive faculties, and actually produce that idea which we then perceive: and we cannot so far distrust their testimony, as to doubt that such collections of simple ideas as we have observed by our senses to be united together, do really exist together. But this knowledge extends as far as the present testimony of our senses, employed about particular objects that do then effect them, and no further. (Bk. 4:11:9)
We cannot demonstrate the necessity of the co-existence of those ideas which constitute the modes or qualities of substances; we cannot perceive their "necessary connexion or repugnancy." The connection between the secondary and the primary qualities remains inexplicable. "And therefore there are very few general propositions to be made concerning substances, which carry with them undoubted certainty" (Bk. 4:6:76). "Our knowledge in all these inquires reaches very little further than our experience" (Bk. 4:3:13-14). Beyond the strict warrant of experience, or the testimony of our senses, we may venture upon "opinion" or "judgment" as to the co-existence of the qualities of substances, but we cannot strictly "know". "Possibly inquisitive and observing men may, by strength of judgment, penetrate further, and, on probabilities taken from wary observation, and hints well laid together, often guess right at what experience has not yet discovered to them. But this is but guessing still; it amounts only to opinion, and had not that certainty which is requisite to knowledge" (Bk. 4:6:13)
Locke finds himself compelled, therefore, to conclude that the so-called "science" of which Bacon had talked so proudly, and of whose achievements he had himself spoken so respectfully in the opening pages of the Essay, is not, in the strict sense, science at all; that, in his own words, there can be "no science of bodies." It is vain to search for the "forms" of the various material substances, or to seek to verify "the corpuscularian hypothesis" as to the connection of the primary and the secondary qualities of things. "I am apt to doubt that, how far soever human industry may advance useful and experimental philosophy in physical things, scientifical will still be out of our reach.... Certainty and demonstration are things we must not, in these matters, pretend to" (Bk. 4:3:26).
If we cannot attain to a science of bodies, still less can we expect "scientifical" understanding of spirits. Spiritual substance is, as we have seen, as unknown as material substance; and Locke finds additional reasons for limiting our knowledge in this sphere.
If we are at a loss in respect of the powers and operations of bodies, I think it is easy to conclude we are much more in the dark in reference to spirits; whereof we naturally have no ideas but what we draw from that of our own, by reflecting on the operations of our own souls within us, as far as they come within our observation. But how inconsiderable rank the spirits that inhabit our bodies hold amongst those various and possibly innumerable kinds of nobler beings; and how far short they come of the endowments and perfections of cherubim and seraphim, and infinite sorts of spirits above us, is what by a transient hint in another place I have offered to my reader's consideration.