In the previous post we argued that as well as evidence, we need a notion of an unjustified rule that takes us from evidence to conclusion. Some may think that this notion is too obscure, or at least philosophically disreputable to do the work it’s put to. This section is then a discussion of rules that aims to increase their respectability. The idea is obviously not new; it traces back as least as far as Wittgenstein on rule-following, if not back to Carroll himself. And it has links to contemporary epistemology, wherever someone says that there are things we are entitled to assume without argument. But since it is doing so much work here, and since in conversation it has often been the most puzzling aspect of the argument to others, it may be worth saying just a little about what I’m taking rules to be here.

The argument in that section relied on Lewis Carroll’s example involving modus ponens, and that might suggest that modus ponens is the kind of rule that we need. Indeed, it often seems that some philosophers think that it’s the _only_ rule we need. (Many philosophers have said that conditionalisation, which is really just a probabilistic form of modus ponens, is the only rule we need.) I think this example is misleading for four related reasons.

First, modus ponens is a rule of *implication* and what we’re really looking for here are rules of *inference*. We’re looking for rules that tell you what to do with evidence. And what you do with evidence is draw inferences from it. These may not, in any interesting sense, be implications of the evidence.

Second, modus ponens is necessarily truth preserving, and this might be thought to be related to its acceptability. This seems like a mistake twice over to me. It’s arguable (indeed I’ll briefly argue for it presently) that we need some ampliative rules to explain the rationality of induction. And there are necessarily truth preserving rules that we cannot employ without justification. The rule that lets us infer _p and oxygen has atomic number 8_ from _p_ is necessarily truth preserving, but not a rule we could freely employ without independent justification.

Third, modus ponens can be used in suppositional reasoning just as well as in regular reasoning. It would take us way too far afield to investigate this properly, but I suspect some of the rules we’ll look at can’t be properly be used in all suppositions. (Of course some rules of _implication_, at least in some formulations, also have restrictions on when they can be used; think of the restrictions on necessessitation or universal-introduction.)

Fourth, modus ponens might (although this is controversial) be constitutively related to the meaning of the conditional. Perhaps, as some inferentialists believe, the meaning is determined by the acceptability of the rule. Perhaps the meaning directly implies that it is an acceptable rule. If either of those things are true, they aren’t I think things we’d want to generalise to, say, rules for rational inductive inference. (There is obviously a lot to be said here, such as considering what might justify inferences related to logic if not meaning. Many recent papers by Boghossian, Williamson, Wright and others are relevant here. But I’m just going to avoid that issue for today, in large part because I do find it mysterious how to generalise much of that debate from issues about modus ponens to issues about, say, enumerative induction.)

Now all that is just to say what rules are not. Can we say what they are? As noted above, in some ways rules are very familiar. Any time any philosopher claims that we are warranted, or entitled, to, without justification, rely on the deliverances of some source, it seems they are proposing that the inference from _The source says p_ to _p_ is a good rule. So we can take dogmatists about perception (e.g. Pryor’s “The Sceptic and the Dogmatist”) as endorsing a rule that lets us move from _Appears that p_ to _p_ without antecedent justification. And some anti-reductionists about testimony seem to hold that the rule which licences the inference from _I’m told that p_ to _p_ is a good one. So in some ways this isn’t a new idea, it’s just a way of framing an old idea. There are, however, two reasons we might think that this is a good framing. First, it lets us ask some relatively precise questions about the statement of the rules. Second, by thinking about rules as a class, we can formulate restrictions on what could be a rule.

It is harder than one might like to actually state rules that we can or do use. Clearly it isn’t a rule that we can, in every case, infer from _Appears that p_ to _p_; there are illusions, some of which we know about. Nor is it a rule that we can, in every case, infer from _I have observed many Fs in widespread conditions, and they have all been Gs_ to _The next F I observe will be G_; there are gruesome predicates. We could try to incorporate the exceptions into the rule, but a quick glance at how one might do this reveals that it isn’t too attractive, at least as long as we want to have non-trivial rules. Probably we need something different.

In the case of appearance, there is a natural move to make. Instead of a single rule with all sorts of qualifications, we might try to defend the following two unqualified rules.

(A) If it appears to you that _p_, then you have a reason to believe that _p_.

(ND) If you have a reason to believe that _p_, and this reason isn’t defeated, believe that _p_

The conjunction of (A) and (ND) entails Pryor’s dogmatism, although that conjunction is considerably stronger than dogmatism. For example, it entails that appearances that you have reason to believe are deceptive provide some reasons; dogmatism as such is silent on such appearances. But both rules seem at least defensible, even given the existence of illusions. (Of course, if they are good rules, they don’t need a defence. But we can reasonably expect that a good rule won’t be such that we can have good reason to believe it is bad, and (A) and (ND) pass that rule.)

The case of induction is a little trickier, because as soon as we think about the rule a hard question arises. We can only project non-gruesome predicates. Does that mean (a) that the non-gruesomeness of the projected predicates should be an input to inferences licenced by the rule, or (b) that the rule requires no such input, but it only licences inferences when the predicates are in fact non-grue? The latter sounds more plausible; otherwise no one who lacked the concept of a gruesome predicate could rationally make inductive inferences. But it raises tricky questions about using that rule in suppositional reasoning. What if (assuming this is possible) different predicates would be gruesome if _p_ were true, and we are inferring under the supposition that _p_? It isn’t obvious just what restrictions should be put on the use of this rule in a suppositional context. Perhaps those restrictions are quite tight. This has important consquences for arguments for the contingent a priori that assume that we can make inductive inferences in suppositional contexts, such as John Hawthorne’s “Deeply Contingent A Priori Knowledge” and my “Scepticism, Rationalism and Externalism”.

That was all about the nature of rules; we might wonder whether there is anything that we can say about which rules there are. Different ways of thinking about Carroll’s example suggest two different constraints on rules, one more liberal and the other more conservative.

The more liberal constraint is a kind of transcendental consideration. In many cases it seems, at least prima facie, that we can get knowledge from a certain source, but we couldn’t antecedently justify the use of that source. A classic example of this kind of reasoning is C.A.J. Coady’s arguments for against reductionism in testimony. The thought there is that so much of the time the only way we have of checking one person’s testimony is through the testimony of another, that if we weren’t able to take some testimony as basic knowledge, we’d be led to a debilitating scepticism. That seems unacceptable, so we might take such a rule as given.

The more conservative constraint takes more seriously the particular way in which we need a rule to sidestep Carroll’s tortoise. The core problem isn’t just that when we add another premise, one that justifies a particular use of the rule, we need yet more to get to the conclusion. Rather, the core problem is that when we add the kind of premise that could justify the rule, we need another step of the very same rule. Justifying this particular use of the rule doesn’t seem to get us any closer to where we need to be. Perhaps those cases, where justifying a particular use of the rule still requires the rule, are the only cases where there are unjustified rules.

This seems to be a more conservative principle because whenever it obtains, we’ll be able to give a transcendental argument for the existence of a rule. But the converse doesn’t seem to hold. We might justify taking someone’s testimony about being true because we believe them to be generally reliable. We still need a rule saying that it’s good to believe those who are generally reliable, but that doesn’t seem like the same rule. Similarly, we might accept (A) as a derived, and hence justified, rule because the best explanation of our experiences is that they are generally reliable. (Jonathan Vogel has argued for this at length over the years.) We still need a rule saying that we should believe the best explanation of a phenomena, but that doesn’t look like rule (A) again. On the other hand, Hume’s arguments about induction arguably do show that a justification of induction will need to use induction. And a justification of (ND) will, I imagine, still use something equivalent to it. So even on the more conservative conception of rules, they may still be rules.

These are enormous questions, to say the least, so I don’t think this goes close to settling anything. But I hope it’s enough to suggest that we haven’t given up on systematic epistemology once we admit the notion of unjustified rules that justify inferences. And admitting that, which Carroll’s example suggests we must, is enough to sidestep Williamson’s argument against EN.