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Locke 1

The Limits of Human Understanding

Having developed all the machinery he feels necessary, in Book IV of the Essay Locke defines knowledge as the agreement or disagreement of our ideas with one another and goes on to explain on this basis what humans are capable of knowing. In Chapter 1 Locke tells us that our knowledge is related to the agreement and disagreement of ideas, and that there are four kinds of ideas:

  1. Identity or diverstiy
  2. Relation
  3. Co-existence, or necessary connection
  4. Real existence
In Chapter II of Book IV he explains that there are some four degrees of knowledge. The first is intuitive knowledge, the kind of thing which we get by the natural light, that two ideas are the same or different is intuitively obvious to us. Nothing could be more certain. Simple ideas are known intuitively. Second, there are truths which are known demonstratively. Demonstration is required when mere juxtaposition of ideas will not show us their agreement or disagreement. In such cases it may be possible to see the agreement by the addition of other intermediate ideas. Demonstrative proofs depend on having an intuitive grasp of the connection between the ideas in each of the steps of the proof. The third level is sensitive knowledge of the existence of external objects. The agreement or disagreement of ideas can be known either intuitively or by reason examining the agreement or disagreement of ideas, or by sensation perceiving the existance of particular things.

In regard to the knowledge of the existance of particular things, a comparison with Descartes is of some interest. Locke writes: "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." He continues: "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." The reason why our knowledge of the existence of external objects is less assured is a certain skeptical challenge: How do we know that there is something in the external world which corresponds to our ideas? In section 14 of Chapter 2 Locke says:

    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.

Thus, as Descartes pointed out, we know for certain that we seem to see material things, but there is a doubt as to whether our ideas correspond to things in the external world. We might be dreaming or being deceived by an evil demon. Still in IV, 2, 14, Locke's claims there is a clear difference between our ideas when we look at the sun by day or remember it later:

But yet here I think we are provided with an evidence that puts us past doubting. 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; when he actually tastes wormwood, or smells a rose, or only thinks on that savour or odour? 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.

So there is a clear difference between ideas of sense and ideas in the imagination. (Hume will later give this difference a name and call ideas of sense impressions and ideas of the imagination ideas. And thus if anyone thinks he might be dreaming, Locke gives this response:

If any one say, a dream may do the same thing, and all these ideas may be produced in us without any external objects; he may please to dream that I make him this answer:- 1. That it is no great matter, whether I remove his scruple or no: where all is but dream, reasoning and arguments are of no use, truth and knowledge nothing. 2. That I believe he will allow a very manifest difference between dreaming of being in the fire, and being actually in it. But yet if he be resolved to appear so skeptical as to maintain, that what I call being actually in the fire is nothing but a dream; and that we cannot thereby certainly know, that any such thing as fire actually exists without us: I answer, That we certainly finding that pleasure or pain follows upon the application of certain objects to us, whose existence we perceive, or dream that we perceive, by our senses; this certainty is as great as our happiness or misery, beyond which we have no concernment to know or to be. So that, I think, we may add to the two former sorts of knowledge this also, of the existence of particular external objects, by that perception and consciousness we have of the actual entrance of ideas from them, and allow these three degrees of knowledge, viz. intuitive, demonstrative, and sensitive: in each of which there are different degrees and ways of evidence and certainty.

So Locke is less inclined to concede as much to the skeptic at the start as Descartes. Still, even though he is sure that material objects exist and cause our perception of them, our knowledge of the nature of material substances is even more problematic than our knowledge that they exist. This brings us to the relation between our ideas of substances and modes and our knowledge of these things.

Recall that our ideas of substances and modes are made by the mind, and that the mind is supposed to copy the external archetype or model, while modes are pefect because they are the standard or archetype to which things must conform. It turns out that because of this difference, there is a correspondingly vast difference between our knowledge of substances and our knowledge of modes.

In regard to substances we are back to the clock at Strasbourg. We have experience of the apparent qualities of medium sized material objects, but we are largely ignorant of the particles which are the cause of our ideas. We don't know how the particles produce they ideas we perceive. If we knew what properties were necessarily connected we might have some chance of having an accurate idea of the essence of particular kind of material substance. But we have very little knowledge of which properties are connected. Our knowledge is limited to our experience of which properties actually appear together. This in its own way is rather unsatisfactory, but the best we can manage in our state.

In contrast our ideas of modes give us certain knowledge. This is because the ideas are not modelled after external archetypes but are themeselves the archetypes. Very likely Locke was thinking of the ideal mathematical forms of triangles, squares and dodecahedrons. Mathemtical objects are modes, and mathematical reasoning is demonstrative. Locke claims that moral knowledge is also capable of demonstration. When we turn to Locke's political ideas, we will have to remember that such terms as justice, rights, slavery, war, political power, and so forth will all count as modes.


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