The companions-in-guilt argument (CGA) should be read more in the direction of, "Unicorns exist, but horses (or goats) don't."𝔘 The metaepistemological version goes something like:
- If moral facts do not exist, then epistemic facts do not exist.
- Epistemic facts exist.
- So, moral facts exist.
- If moral facts exist, then moral realism is true.
- So, moral realism is true (Cuneo 2007: 6)
The argument is deductively valid (assuming modus ponens, anyway), so the soundness of (1) is, let us assume, what is at stake in the CGA, here. But we must distinguish between instrumental and categorical epistemic reasoning, and then question the categorical value of the instrumental principle in itself. It is easy to accept a hypothetical imperative such as, "If your goal is to believe validly deduced conclusions, then conform your reasoning to the principles of validity," but then we will wonder what those principles are, whether they are "realistic" or not. And what if one's goal is to believe on the basis of other principles of validity? One will object that there are no such things: that truth is true, but ought to we to accept the truth? "If you don't accept the truth, your plans will go awry": perhaps, perhaps not. Foolish and dishonest people have a remarkable success rate in this world, after all. And if everyone has a problem with being self-deceptive, then anyone who is successful is so despite their self-deceptions, and we will wonder all the more whether commitment to believing the truth is more than instrumentally valuable?
𝔘If unicorns are impossible, this might not be the most perspicuous analogy anyway, though.
ADDENDUM: the knowability principle and metaepistemic normativity, and the puzzle of epistemic obligation
Suppose that knowledge is normative, to wit if t is any truth, then t → OBKt.D If it is obligatory that all truths are known, then it is possible that they all are, but then we have the knowability principle such as Fitch's result tells against.
Or then consider the puzzle of epistemic obligation in general. If there are truths that we are obligated to not know, what becomes of epistemic normativity then, as a species (the only living one, you have said) of moral normativity? We should like to find a way to talk about an obligation to know that something wrong has been done without there being an obligation to do the wrong thing (that it would be known), but so far a conclusive formalization of this discrepancy escapes us. Is epistemic normativity a default companion in that guilt, though, then?
DHowever, we might try out a different principle instead, here. For example, we might say that if it is possible to know whether t, then it is obligatory to know whether t: ◊Kwhethert → OBKwhethert. Yet in the quest for knowledge, then, it seems we will end up being obligated to do certain things to know whether t, for example if we ought to know whether someone is suffering, then we will be obligated to empathize with them, etc. When epistemology is socialized, the normativity of sociability will be a part of the normativity of knowledge (c.f. Kant's maxim of reason: "Think oneself into the place of others").