I gain evidence that this used car salesman is unreliable. He tells me he’s the most honest person I’ll ever know. It would be ridiculous, on that basis, to think “Well, gee, I guess he’s reliable after all.” We need independent confirmation that his testimony is reliable. Here are two more controversial examples. I gain evidence that my moral intuitions are unreliable. Can my moral intuitions defeat that evidence? It seems not: to defeat that undercutting evidence, you would need to appeal to something independent of the intuitions undercut. You’re smart, but we disagree about P, which gives me some reason to think that my relevant arguments and intuitions aren’t reliable. Can I rely on those intuitions and arguments to defeat the evidence of my unreliability? It seems not: again, we seem to think we need something independent of those arguments and intuitions.
Our intuitions suggest that, to defeat undercutting evidence, we need to appeal to something independent of what’s undercut. I think those intuitions are wrong. Suppose I receive testimony from a reliable source that all my belief-forming methods are unreliable. Given our “independence intuitions”, this undercutting evidence is impervious to defeat. Since it undercuts everything, there is nothing to which I can appeal. Yet the mere generality of the defeater should not make it invincible.
When we realize that the intuitions are wrong, we have some explaining to do. If undercut evidence can defeat the undercutting evidence, then what’s the problem with trusting the testimony of the car salesman? And what’s the problem with relying on the wall’s appearing red to give me evidence of my reliability in the context where the wall is lit by red lights? If our “independence intuitions” don’t explain what’s going on in these cases, why can’t we appeal to the undercut evidence to defeat the undercutting evidence?
Feel free to comment on the above quick and dirty argument without consulting the material below the fold. For those who want to see the argument against our independence intuitions laid out more carefully, see below.
I’ve just completed a mathematics test. My belief in each answer is highly justified. But then I’m told by a reliable source that my coffee was laced with a drug that makes mathematical reasoning highly unreliable for at least four hours (it only took 1 to complete the test). It seems that I should now lower my confidence that my answers are correct. More generally, when our beliefs are highly justified (but not certain), and we acquire some undefeated evidence that they were formed in an unreliable way, it seems that our beliefs are less justified and, perhaps, no longer justified at all. More formally, we can appeal to the:
Undercutting Evidence Principle (UEP): If my beliefs in domain D are highly justified (but not certain) at t1 and if, at t2, I acquire evidence that my judgments within D were formed using method M and that M is unreliable, then, unless this evidence is defeated, my beliefs in D are less justified at t2 than they were at t1.
Once I’m informed of the drug, I begin to check my answers. I consider first question. I check my calculations and they seem flawless. I then realize that there are other ways of calculating the answer to the question. For example, instead of multiplying 4 x 4, I add 4 + 4 + 4 + 4. I consider an additional five ways of calculating the results to the answer and each time I confirm my original answer. I undertake a similar investigation for each of the other 9 answers. My new calculations confirm my old ones again and again. Presumably, these confirmations, when taken together, count as evidence for the reliability of my mathematical reasoning. Can this evidence raise my justification that my answers are correct (at least partially) back to their original level? Or perhaps even higher than their original level? Intuitively, many think the answer is clearly no. These “confirmations” appealed solely to mathematical reasoning. But that is the very sort of reasoning that my evidence suggests is unreliable. How can further investigation defeat the undercutting evidence by relying on the very sort of reasoning that is undercut? At first glance at least, that’s just plain foolish. Hence, the following principle may seem undeniable:
Independence: A body of evidence E* can defeat undercutting evidence that M is unreliable only if E* was, at least in part, was not obtained via M.
Unfortunately, things aren’t so simple. We are faced with a contradiction when we consider two other very plausible claims. Consider first:
Possibility of Global Undercutting (PGU): it is possible to have beliefs that are highly justified at t1 and then acquire evidence at t2 that all of your belief forming methods are unreliable, so all your judgments are/were formed using methods.
PGU is a very weak claim. It says that it is possible to have beliefs that are highly justified and then acquire evidence that all your beliefs are formed via some unreliable method. Suppose, for example, that I receive reliable testimony that all my belief-forming methods are unreliable. This testimony is confirmed by the world’s leading experts. The most reliable news outlets testify that my beliefs are unreliable, an unfortunate result of an insidious science experiment. I would say that I have now acquired evidence that all my beliefs are formed by some unreliable method. Indeed, one might think that this is fairly strong evidence that all my belief forming faculties are unreliable. I usually confidently believe things on far less impressive testimony.
But when you put UEP and PGU together, it follows that I should reduce confidence in all my beliefs. Is there any way that I can regain the lost degree of justification? Not given Independence. Since it disallows me from appealing to anything undercut and every way I have of forming beliefs is undercut, there is nothing to which I can appeal. I’m stuck where I am—at least until I gain even more evidence of my unreliability, at which point my justification might decrease even further. Hence, invincible counterevidence would be possible. Yet the mere fact that undercutting evidence is completely general should not make that evidence invincible, as would be the case were Independence true. In other words:
Anti-Invincibility: A body of undercutting evidence E can’t be invincible, i.e. there must be some possible circumstances in which E is defeated. Or at the very least, the generality of the undercutting evidence shouldn’t ensure that the defeater is invincible.
Setting aside the possibility that one can acquire new belief forming methods, UEP, Independence, PGU, and Anti-Invincibility form an incompatible tetrad. UEP, Independence, and PGU guarantee that invincible defeaters are possible, and Anti-Invincibility guarantees that there aren’t any.
I take it that the two least plausible claims are Independence and Anti-Invincibility. Which one should we give up? Which one is the least plausible? I don’t have a super strong argument for Anti-Invincibility, but I do want to put a little pressure on Independence. In the case described above, I have all sorts of testimonial evidence that suggests all my belief forming methods are unreliable. Independence entails that, even if I were to acquire rather convincing evidence that the whole thing is an elaborate gag, this evidence still could not restore me to my original level of justification. And that seems rather counterintuitive, and it reinforces the intuition that the mere generality of undercutting evidence should not make it invincible. I think, therefore, that we should reject Independence.