Hume’s predicament, probabilism and idealism 79 this is a legitimate use or extension of the word “know” is a question that

can be set aside. Popper’s substantial point is that the relevant epistemic category for both our (good) ordinary and scientific beliefs is one that includes false beliefs. There is no epistemically relevant category oftrue scientific beliefs, for instance. This sets Popper’s view apart from the later justified true belief analysis of Chisholm’s and Ayer’s. And this puts Pop-per closer to Peirce and to Carneades. PopPop-per’s notion of “knowledge”

is a (significantly refined and updated) version of Carneades’s notion of probable opinion.

C. I. Lewis (1929) endorses the idea that all beliefs about empirical matters of fact are probable only. But he nevertheless calls them “knowl-edge”, in violation of the dichotomy between certain knowledge and probable opinion endorsed by the Stoics, the Sceptics and Locke:

The only knowledge of a priori is purely analytic; all empirical knowledge is probable only. In affirming such a view, one assumes a heavy burden of proof; the whole history of the theory of knowledge (unless we go back to Plato) seems to enforce the conclusion that such a conception must inevitably lead to scepticism. Lewis(1929, 309)⁵⁸

For Lewis, Hume’s sceptical arguments establish that empirical knowl-edge is at most probable (Lewis, 1929, 323). On his view, any statement about empirical matters of fact implicitly asserts something about “all future possible experience”, and there is always a possibility of these assertion to turn out false (Lewis, 1929, 280–1). But what does Lewis mean by probableknowledge? He does not mean by it, as one would have expected, knowledge on a fallible basis. Rather, he means knowledge of probabilities. On Lewis’s view, we do not know that, say, there is a tree in the garden, but we know that there isprobablya tree in the garden:

are falsified, hence the condition of falsifiability is compatible with factivity. My (ill-formulated) point here is that some falsifiable hypotheses haveturned out to be false, and yet as far as I can see, Popper would still count them as instances of scientific knowledge. And to my knowledge, Popper never insists on truth being a condition on scientific knowledge.

58. I do not know on what grounds C.I. Lewis thinks that Plato shared his view.

if we thus state, for example, Newton’s law of gravitation as an absolute truth, we must not confuse what is stated with the judgement of any informed and intelligent person who makes the statement. The intent of the judgement is not the statement judged probable, butthatit is probable. If in such a case we assert briefly that A is B, our judgement is, “It is probable that A is B” [. . . ]. (Lewis,1929, 324–325)

The motivation for Lewis’s view is clearly that by switching to contents of the form probably p, he hopes to restore the infallibility of empirical knowledge. First, he points out that probably pmay be true even ifp is false:

Now a common supposition seems to be that our knowledge of the law of gravitation is invalid if there are facts of nature which do not conform to the law. But if this is probable knowl-edge, it is a very simple and obvious fact that its validity does not require such conformity. The judgement “A is B is proba-ble” does not require for its truth that A is B; it requires only that this should be genuinely probable. (Lewis,1929, 324–325) And secondly, he argues that a proper probabilistic inference preserves the truth of its conclusion:

The probable judgement, if valid, is true. There is no difference in the case of probability-inference between validity and truth.

What the judgement “A is probably B” asserts is not that A is B or that any other objective state of affairs (except what the premises assert) holds good. It asserts that “A is B” has a certain probability on the basis of certain data. (Lewis, 1929, 331)

Lewis’s epistemology thus fully conforms to the Ur-foundationalist pat-tern. We have basic knowledge of sense-data, which serve as their own infallible and discernible marks. From this we derive knowledge through truth preserving inferences. But the major concession to scepticism is that we do not reach knowledge of facts we would ordinarily take ourselves

2.4. Hume’s predicament, probabilism and idealism 81 to know. We only learn that certain claims are probably true according

to the data we have. Carneades “convincing” impressions are recast into deductive proofs of probabilistic claims.

(Some remarks about Ayer and Russell. Ayer (1936/1990, 19) seems to have endorsed a view in Carneades’s tradition: “Indeed, it will be our contention that no proposition, other than a tautology, can possibly be anything more than a probable hypothesis.” He does not use the term knowledge, so it is hard to say whether he had a view like Peirce’s or like C. I. Lewis’s in mind. In spite of its title, hisFoundations of Empirical Knowl-edge(Ayer, 1940) hardly mentions knowledge either. In it he restates the view that any empirical belief has a merely fallible basis (Ayer,1940, 39, 43). His main claim is that empirical beliefs can be based on “reasonable”, though not “demonstrative”, inferences (see e.g. 230). Since he grants that these claims may turn out false, he appeared to endorse a version of Carneades’s scepticism, like Peirce, rather than C. I. Lewis’s infallibilist probabilism.

Russell’s analysis of knowledge in Human Knowledge (1948) is not altogether clear to me. He puts forward the “true belief supported by adequate evidence” account with some reticence (Russell, 1948, 170–1).

The whole thrust of the book is that Humean scepticism is averted once we recognise that there are grades of knowledge and that these grades are degrees of probability. My best guess is that he has a view like that of Lewis’s in mind.)

2.4.3 Idealism

A third Ur-foundationalist reaction to sceptical arguments isidealism and its more modern cousin, verificationism. I am not claiming that the difficulties of dogmatic Ur-foundationalism are the only reason for the de-velopment of idealist views from Berkeley on. But they certainly played a role.

Locke’s main answer to the sceptical claim that perceptual impressions cannot be discerned from dream ones was an outright denial, as we have

seen (2.4.1). But right after, he sketches an idealist response, in case his denial is not granted:

But yet if [one who argues that a dream may produce the same idea] be resolved to appear so sceptical 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. (Locke,1975, IV, 2, §14)

The rejoinder appears to be the following: even if what I call “fire” turns out to be an idea in me, it is no less a cause (or temporal predecessor) of pain, and that is all that matters to us. The suggestion is that (1) even if I am fallible about the existence of fire, I am at least infallible about the existence of my sensation of fire, (2) and perhaps this is all the knowledge I need. The further step, taken by Berkeley (1710, §1) and subsequent idealists, was to claim that “fire” in fact refers to these sensations. We thus turn out to have infallible beliefs about fire itself. Dogmatic Ur-foun-dationalism is restored through the concession that knowledge is about our ideas only.⁵⁹

The dogmatic Ur-foundationalist motivation behind idealism is evi-dent in Berkeley’s and Kant’s case. Berkeley(1710, §18) grants sceptical arguments concerning the existence of things without the mind: sensa-tions are infallible and discernible marks of themselves only, and since there is no necessary connexion between things outside and sensations inside (as dreams show), any inference from the latter to the former is fallible. But instead of concluding that we do not know, he concludes that the things we know about are ideas.

59. Locke himself says that our knowledge is “conversant about our ideas only” (IV, 1, §1), but this claim is notoriously hard to interpret (seeNewman,2007, 315–7 for some discussion and references).