Today’s question: If induction is how we know the future, but induction cannot be justified without assuming itself — what are we actually doing when we “know”?
After yesterday’s reading, I was concluding that reflection could only follow from extensive sensation that occurred during early development — that reflection, instead of being a type of first-order thought, was in fact derivative of sensation. But Locke’s deeper problem, the one Hume targets, runs further in. His system assumes, without quite saying so, that the regularities experience has shown us in the past reliably justify our expectations of the future. In today’s reading (Section IV of Hume’s Enquiry), Hume addresses this with care and precision. His target is not empiricism itself, but rather the idea that empiricism can be given a rational foundation — that the lessons of experience can be justified, rather than merely trusted. The damage Hume does is surgical, not total — and as he himself points out later in the reading, it is to be celebrated as an elevation of the empiricist argument, not a denigration of it.
It is interesting that Section IV begins immediately by asserting something that requires a shared assumed framework in order to function — namely mathematics. Hume says that all objects of human reason or inquiry may be naturally divided into two kinds, but I’m not sure I believe that framing. As I discussed in yesterday’s reading, matters of fact are what lead to relations of ideas, not the other way around. There is, I would argue, no such thing as truly a priori knowledge — everything comes from our experience, and early childhood development makes this clear. Three times five is equal to half of thirty, or Pythagoras’ theorem. But those statements can only be declared true and agreed upon once the concepts of numbers, counting, and relationships are already understood — and those are not the first things we, as humans, come to understand. Relations of ideas are not as atomic, or as fundamental, as Hume’s framing suggests.
This distinction — between relations of ideas and matters of fact — carries more weight than it might first appear. Hume’s requirement that a matter of fact must have a conceivable negation is, in structural terms, precisely what Karl Popper would formalise as falsifiability almost two centuries later. For Popper, a claim counts as scientific only if there exists some observation that could, in principle, prove it wrong. Hume is making the same demand in philosophical register: if you cannot even imagine your claim being false, it is not a claim about the world — it is a claim about the definitions you are using. The sciences have since made this their operating assumption so thoroughly that we rarely notice it came from anywhere. It came from here. Hume himself frames the discovery of such defects as an incitement rather than a discouragement — which is, of course, precisely the spirit in which modern science operates.
Hume is equally careful when he introduces causality. He says that all reasonings concerning matter of fact seem to be founded on the relation of cause and effect — not that they are. The hedging is deliberate, and it mirrors the modern scientific method, where nothing is certain and everything is a theory pending disproof. It comes back to inference: our sense of causality is grounded in experience. A voice in the darkness leads us to infer another person — not because reason demands it, but because experience has taught us that voices come from people. That inference could be wrong. The voice could be a recording. Our cause-and-effect reasoning is, at bottom, a series of best guesses calibrated by previous experience.
Hume proves, almost casually, that causality itself is a best guess. It is possible — implausible, but possible — that two very different causes produce a similar effect, or that two nearly identical causes produce very different ones. His billiard ball example is well chosen: we know how complex the physics of billiards actually is. Angles, materials, environmental conditions, the force and precise point of the cue’s strike — all of it matters. And beyond all of that, there remains the vanishingly small but non-zero possibility that some piece of cosmic matter intercedes at precisely the right moment to alter the interaction entirely. Is it plausible? Very much not. But is it possible? Yes. We have seen stranger things.
Consider bit flipping in a processor, caused by alpha particles emitted from trace impurities in chip ceramics. The likelihood of any given bit flipping from this cause is infinitesimal — yet it happens, and when it does, it appears to occur without cause. There is a cause; we simply cannot identify it at the moment of occurrence. Hume raises gravity and elasticity as the apparent limits of what we can trace causally. We now know that is not the limit at all. Special relativity, general relativity, quantum particle physics, the interactions between phases and layers of matter — each level of inquiry reveals further structure beneath. And we know we are still skimming the surface. Causality as a general framework has yet to be disproved; we simply continue to discover that we had not understood the causes correctly. We must be comfortable with that uncertainty. It is not a weakness of the framework — it is the framework being honest about itself.
As I argued at the outset, this is not a refutation of empiricism. It is a clarification of what empiricism can legitimately claim. Hume does not prove that cause and effect are fictions — he proves that they are inferences, and that the justification for those inferences cannot itself be empirical without circularity. What we are left with is not sceptical paralysis but a more honest account of what knowledge actually is: a set of habits, calibrated by experience, that serve us well enough to build chip architectures and predict particle interactions, but that do not carry the metaphysical guarantee that rationalism wanted them to have. The empiricist project survives Hume. Its pretensions do not.
There is a moment in Section IV — around paragraph 26 — that reads, if you are paying attention, as something rather pointed. Hume observes that we cannot posit a cause we cannot trace: where our causal chain runs out, our inference must stop. We cannot leap from “I cannot identify the cause” to “therefore the cause is X.” The logical move is prohibited by the very framework he has just established. It is not hard to see who he has in mind. The God of the gaps — the explanatory deity who fills whatever our science cannot yet reach — is precisely the sort of posited cause that Hume’s system will not licence. He does not say so directly. Neither did Locke. Yesterday’s reading was notable for how long Locke spent on the soul, on the nature of God, on questions whose urgency is difficult to feel now that empiricism has largely won. That circumspection was not accident or piety — it was pragmatism. Both men were writing in an intellectual culture where the Church was not merely a social institution but an epistemic authority, and direct challenge carried real risk. Hume is more pointed than Locke, but he is still pointing obliquely. The argument does the work; he lets the reader draw the conclusion.
There is, of course, something worth acknowledging in all of this: much of what Hume establishes feels self-evident now because it has become self-evident. We stand on the shoulders of these thinkers, which is precisely why looking down feels so natural.