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A. Living by Reason

The initial problem, of course, is that disorderly crowd of opinions: “men, extending their inquiries beyond their capacities, and letting their thought wander into those depths where they can find no sure footing, it is no wonder that they raise questions and multiply disputes, which, never coming to any clear resolution, are proper only to continue and increase their doubts, and to confirm them at last in perfect skepticism” (Locke’s Introduction to the Essay, para. 7, p. 31). Like Hume and Kant after him, Locke thinks the remedy requires that we first make a juster and more accurate appraisal of our intellectual capacities and capabilities:

Whereas were the capacities of our understandings well considered, the extent of our knowledge once discovered, and the horizon found which sets the bounds between the enlightened and dark parts of things; between what is and what is not comprehensible by us, men would perhaps with less scruple acquiesce in the avowed ignorance of the one, and employ their thoughts and discourse with more advantage and satisfaction in the other. (Locke’s Introduction to the Essay, para. 7, p. 31)

The aim is not to achieve Cartesian certainty (about which he makes several disparaging remarks). Rather, “If we can find out those measures, whereby a rational creature, put in that state in which man is in this world, may and ought to govern his opinions, and actions depending thereon, we need not to be troubled that some other things escape our knowledge” (Introduction, para. 6, p. 31). What we need to find out is how we may and ought to govern and regulate our opinion, or assent. And his answer, in prototypical Enlightenment fashion, is that we ought to govern our opinion by following reason. But what does that mean? What is opinion and what is reason, and how can we govern the former by following the latter?

1. Opinion

If we are to have any hope of overcoming the contentious and disputatious horde of conflicting opinion with which we are beset, says Locke, we must all learn to govern opinion and assent properly. Following Plato, Locke thinks of opinion as contrasting with knowledge; to see what he thinks opinion is, we must therefore look to his views about knowledge. He thinks we have four kinds of knowledge, all of them involving certainty. First, there is what he regards as the paradigm of knowledge: perceiving the “agreement or disagreement of our ideas.” It isn’t easy to see precisely what he had in mind here, but the principal sort of knowledge involved here is the knowledge of 76self-evident propositions, such propositions as 2 + 1 = 3.6767   For an account of self-evidence, see WPF, chapter 6. A properly functioning human being can simply see that these propositions are true (and further, that they couldn’t possibly be false). There is no issue of regulating this kind of belief, says Locke, because a properly formed human being simply can’t withhold belief from self-evident propositions: “This part of knowledge is irresistible, and like bright sunshine, forces itself immediately to be perceived, as soon as ever the mind turns its view that way; and leaves no room for hesitation, doubt, or examination, but the mind is presently filled with the clear light of it” (IV, ii, 1, p. 177). Such knowledge is certain; it is “beyond all doubt, and needs no probation, nor can have any; this being the highest of all human certainty” (IV, xvii, 14, p. 407).

Second, there is knowledge of propositions about the contents of your own mind, that is, propositions about the ideas of which you are the subject. An example would be your knowledge that you have a mild pain in your left elbow, or that you seem to see something white (i.e., things look to you the way they look when you are in fact seeing something white). This knowledge, says Locke, is infallible (IV, i, 4, p. 169, and elsewhere). This means at least that you cannot mistakenly believe such a proposition; if you believe that you seem to see something white, it follows that you do seem to see something white (though, of course, you may be mistaken in thinking there really is something white there). Following later custom, let’s say that propositions of this sort about my own mental states are incorrigible for me.

Third, there is also a kind of knowledge of “other things,” of external objects around you:

And of this, the greatest assurance I can possibly have, and to which my faculties can attain, is the testimony of my eyes, which are the proper and sole judges of this thing; whose testimony I have reason to rely on as so certain, that I can no more doubt, whilst I write this, that I see white and black, and that something really exists that causes that sensation in me, than that I write or move my hand; which is a certainty as great as human nature is capable of, concerning the existence of anything, but a man’s self alone, and of God. (IV, xi, 2, pp. 326–27; see also IV, ii, 14, p. 186)

It isn’t wholly clear just what it is I know here: do I know that the piece of paper is white, that my hand is moving, and that the ink is black? Locke vacillates. Sometimes (for example, when commenting on the relation between faith and reason) he speaks as if our knowledge of external objects includes the sort of everyday knowledge we get from perception: that my hand is moving, that the trees in the backyard are budding, and so on. Other times, and perhaps when 77he’s being more careful or at least more official, he suggests that what we know of the external world is much sparser, more like My current ideas of treehood and green are caused by something external to me. I may not know what these external objects are like (I don’t know that they include trees, or buds, or objects that are green), but I do know that there is something external causing me to have these ideas.

And fourth, there is demonstrative knowledge. I can come to know a proposition by deducing it from or seeing that it is entailed by propositions of the above three sorts (where a proposition p entails a proposition q just if it is not possible, in the broadly logical sense, that p be true and q false.6868   I do not and cannot know all propositions entailed by those of the above sort, of course; some might be much too complicated and difficult for me to grasp, and others might be such that I simply can’t see the connection between them and propositions of the above three sorts. For still others, the argument for them is so long and complicated that I lack the certainty required by knowledge. Accordingly, some propositions that you can deduce from propositions that are self-evident, incorrigible, or evident to the senses are also certain for you; among these propositions, Locke thinks, is the existence of God (IV, x, 1–6, pp. 306–10). Indeed, he adds, “From what has been said, it is plain to me we have a more certain knowledge of the existence of a God, than of anything our senses have not immediately discovered to us.”

When it comes to knowledge, therefore, we have no control over our giving assent; assent is elicited willy-nilly, and the question of how we should regulate assent in this area therefore does not arise. (We can’t regulate it at all, anymore than I could regulate the direction in which I fall, if I fell off a cliff.) Of course, knowledge forms only a part of the beliefs to be found in a human noetic structure and, according to Locke, a relatively small part (“Our knowledge, as has been shown, being very narrow,” IV, xv, 2, p. 364). It is opinion that includes the bulk of what we ordinarily believe; and it is with respect to opinion—that which we believe but do not know—that the question of regulation does arise.

2. Reason

Locke’s crucial claim is that we must be guided, in the formation of opinion, by reason. Well, what is reason? First, it is “a faculty in man, that faculty whereby man is supposed to be distinguished from beasts, and wherein it is evident he much surpasses them” (IV, xvii, 1, p. 386). Second, reason is the power whereby we can discern broadly logical relations among propositions (IV, xviii, 3, p. 417), which, of course, are the candidates for our assent, the things we believe. In particular, by virtue of employment of reason, we distinguish two kinds of relations among propositions:


The greatest part of our knowledge depends upon deductions and intermediate ideas: and in those cases where we are fain to substitute assent instead of knowledge, and take propositions for true, without being certain they are so, we have need to find out, examine, and compare the grounds of their probability. In both these cases, the faculty which finds out the means, and rightly applies them, to discover certainty in the one, and probability in the other, is that which we call reason. For, as reason perceives the necessary and indubitable connexion of all the ideas or proofs one to another, in each step of any demonstration that produces knowledge; so it likewise perceives the probable connexion of all the ideas or proofs one to another, in every step of a discourse, to which it will think assent due. (IV, xvii, 2, p. 387)

It is by reason, therefore, that we perceive deductive and probabilistic relations among propositions. We needn’t say anything here about deductive relations between propositions; and while much needs to be said about probability, Locke doesn’t say it. He does say a little, however, beginning rather inauspiciously by declaring, “Probability is likeliness to be true” (IV, xv, 3, p. 365). The uninformative character of this, however, is ameliorated by his pointing out6969   As Aristotle also did; see WPF, p. 159. that probability has to do with what occurs ‘for the most part’ in our experience; and he adds that testimony from others also establishes probability (IV, xv, 4, pp. 365–66). Locke seems to think of probability as an objective relation among propositions; he probably also thinks that it is a quasi-logical relationship among them; his views, therefore, may be precursors of those of J. M. Keynes, Rudolf Carnap, and others.7070   What he says is consistent with other views, however, including the one proposed in chapter 9 of WPF. Probability, furthermore, comes in degrees: “Upon these grounds depends the probability of any proposition: and as the conformity of our knowledge, as the certainty of observations, as the frequency and constancy of experience, and the number and credibility of testimonies do more or less agree or disagree with it, so is any proposition in itself more or less probable” (IV, xv, 6, p. 367). Here he seems to suggest that a proposition is probable to some degree “in itself”; he is better understood, I think, as holding that probability is a relation between propositions. A proposition has a certain degree of probability ‘for me’ (i.e., relative to those propositions that are certain for me); what counts with respect to the formation of my opinion is the probability of the candidate in question with respect to what is certain for me.

3. Regulating Opinion by Reason

Locke’s claim is that we should regulate our opinion or assent by reason; but what does this mean? How do you do a thing like that? His answer, fundamentally, is that I must regulate my opinion in such a way that I opine only that which is probable with respect to that which is certain for me. I have no control over my assent when it comes to knowledge, what is certain for me; however, I do have control over my assent when it comes to opinion, what isn’t certain. And the rule here is that I must not assent to a proposition unless it is probable with respect to what is certain for me. Assent, furthermore, comes in degrees7171   Here I think he means to point to two phenomena: first, that one believes some propositions more firmly than others, and second, that we judge some propositions more probable than others. To illustrate the first, I believe that 7 + 5 = 12 more firmly than I believe that Glasgow is west of Aberdeen, but I do believe both of these propositions. As for the second, I believe it is reasonably probable that all the continents of Earth once formed a supercontinent; I also believe that it is more probable that the works attributed to Shakespeare were really written by Shakespeare, not by Bacon. (IV, xvi, 1, p. 369). More exactly, then, the rule is that I should proportion my degree of assent to the probability of the proposition in question: “The grounds of probability we have laid down in the foregoing chapter: as they are the foundations on which our assent is built, so are they also the measure whereby its several degrees are, or ought to be regulated” (IV, xvi, 1, p. 369). More specifically, for any proposition that comes to my attention, I should proportion my degree of assent to it to the degree to which that proposition is probable with respect to what is certain for me. Proper procedure here is “not entertaining any proposition with greater assurance than the proofs [deductive or inductive] it is built upon will warrant” (IV, xix, 1, p. 429) (probabilistic proofs as well as deductive proofs). Another way to put this: I should proportion degree of assent to the evidence; that is, I should believe a proposition p with a firmness that is proportional to the degree to which p is probable with respect to what is certain for me. This is what it is to regulate or govern opinion according to reason.

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