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Borrowing heavily from Plantinga, I’ll use the drug XX analogy
[4], where drug XX is a fictional drug that renders one’s cognitive faculties unreliable for the vast majority of those who take it, with the type of unreliability in question being general cognitive unreliability, such that those so afflicted can’t even rely on their cognitive faculties to determine whether they’ve passed cognitive reliability tests and aren’t necessarily capable of detecting their own cognitive unreliability (note that this is the same type of unreliability that EAAN has in mind), though those afflicted can still have some true beliefs, including the belief that drug XX entered their system (though they might be mistaken how they came to that belief). The only people who are immune to drug XX are those that have the blocking gene, a gene that produces a protein that blocks the effects of drug XX, but the likelihood of any given individual having the blocking gene is small. Once a person ingests drug XX, the drug soon enters the bloodstream and has a high probability of rendering that person’s cognitive faculties unreliable within two hours. With that, consider the following scenarios:
Scenario (S1): I know that my friend Sam ingested drug XX and that twenty-four hours later he came to believe that that a series of tests has confirmed that he has the blocking gene and that his cognitive faculties are reliable, though I have no independent reason for thinking this occurred. And since Sam obtained his belief about the cognitive tests long after he ingested drug XX, there’s a reasonable chance that this belief was produced by unreliable cognitive faculties, and so this would-be evidence for Sam’s cognitive reliability (Sam’s memory of passing the cognitive tests) is undermined by drug XX, and my belief
Drug XX entered Sam’s bloodstream defeats my belief that Sam’s cognitive faculties are reliable.
Scenario (S2): After I learn about poor Sam I ingest drug XX while being aware of its potential effects. I know of no relevant difference that distinguishes my case from Sam’s. Some years after the incident I come to believe I have taken a series of tests that say I have the blocking gene and that my cognitive faculties are reliable, but since this belief came long after I ingested drug XX, it seems this would-be evidence for my cognitive reliability (my memory of passing the cognitive tests) is undermined by drug XX, just as the would-be evidence for Sam’s cognitive reliability (Sam’s memory of passing the cognitive tests) is undermined by drug XX in (S1). Thus my belief
Drug XX entered my bloodstream defeats my belief that R is true with respect to me.
The above two scenarios illustrate how drug XX can defeat R even for oneself, and also provide an undefeated defeater, since any alleged evidence one comes to believe in after ingesting drug XX would be undermined by drug XX. In scenario (S2), any evidence I give for the reliability of my cognitive faculties (e.g. my memories) would be presupposing the accuracy of my cognitive faculties—thus being circular reasoning. I consequently can’t maintain my belief in R in (S2) on the grounds that my cognitive faculties seem reliable to me, or that nothing seems to have changed for me, because all that relies on the very cognitive faculties that are being called into question. If I had knowingly taken a drug like drug XX that has a high probability of rendering my cognitive faculties unreliable, I can’t legitimately use something that’s the product of those cognitive faculties as evidence for my cognitive faculties being reliable. In scenario (S2), I cannot reasonably believe R is true for me while believing I ingested drug XX, and thus my belief
Drug XX entered my bloodstream defeats R with respect to me.
Plantinga suggests N&E is like drug XX in its power to defeat R. N&E, like drug XX, plays a causal role in cognitive reliability; on N&E, it is naturalistic evolution that created humans and their cognitive faculties, so if on N&E it is likely that naturalistic evolution gave us unreliable cognitive faculties, that would seem to provide a defeater for R. Not only that, but Pr(R|N&E) being low would seem to provide an undefeated defeater for R.
While I think scenarios (S1) and (S2) illustrate the general principle, I think we can make the case stronger using some scenarios from a scholarly paper titled
A Mental States Approach to the Evolutionary Argument Against Naturalism. Let’s consider a few more analogies, some of which refer to the XX-mutation—a genetic mutation that injects drug XX into the bloodstream soon after one is born.
Scenario (S3): A doctor has injected me with drug XX soon after I was born (the doctor mistakenly thought he was injecting an important vaccine), and I come to believe in the following. I initially believe I am the product of a sort of evolution that makes the reliability of my cognitive faculties very likely. I am a renowned scientist who has built a machine that I know is capable of reliably detecting whether and when drug XX entered a person's bloodstream, and I am extremely confident about the reliability of this machine (I as a qualified expert see that it works for myself and numerous scientific experts have unanimously agreed that it is reliable), such that if the machine reports drug XX entered my bloodstream, I would be as confident that the drug did enter my bloodstream as I would be in Scenario (S2). I administer the test to myself and the machine reports that drug XX entered my bloodstream at around the time I was born; as such, I am as confident that drug XX entered my bloodstream as I am in scenario (S2). Later I come to believe I have taken an extensive battery of cognitive reliability tests to confirm that I have the blocking gene, but since this belief came long after drug XX entered my bloodstream, it seems this would-be evidence for my cognitive reliability (my memory of passing the cognitive tests) is undermined by drug XX just as it is in scenario (S2), and so it seems my belief
Drug XX entered my bloodstream soon after I was born defeats my belief that R is true with respect to me.
Scenario (S4): I come to believe in the following. The XX-mutation afflicts approximately one in a million individuals, with only a small percentage of those with the XX-mutation having the blocking gene. I have constructed a device similar to the one described in (S3) except this device detects whether evolution gave someone the XX-mutation, and I am as confident in the reliability of this machine as I am with the one in (S3). For most of my life I believe that I am the product of naturalistic evolution that makes my cognitive reliability very likely. After some years though I finally try the XX-mutation detector on myself. To my horror, the machine reports that I have the XX-mutation and thus that drug XX entered my bloodstream soon after I was born, thereby making me believe that naturalistic evolution gave me the XX-mutation. Later I come to believe that I’ve passed a series of cognitive tests to confirm that I have the blocking gene, but since I believe these tests happened long after drug XX entered my bloodstream, it seems that this would-be evidence for my cognitive reliability (my memory of passing the cognitive tests) is undermined by drug XX just as it is in scenario (S3), and so it seems that I have a defeater for my belief that my cognitive faculties are reliable. My belief
Drug XX entered my bloodstream soon after I was born defeats R for me here just as it does in scenario (S3). Similarly, my belief that
I have the XX-mutation (since I believe this mutation injects drug XX into my bloodstream soon after I’m born) defeats my belief that R is true with respect to me.
Scenario (S5): I come to believe in the following. Via a nifty combination of scientific and philosophical argumentation, it is proven beyond all reasonable doubt that naturalistic evolution entails that the XX-mutation is inevitably a part of any humanoid’s genetics. The aforementioned scientific and philosophical argumentation say that given N&E, it is likely that the XX-mutation rendered everyone’s cognitive faculties unreliable, though on N&E there is also the small chance that everyone evolved the blocking gene to render everyone immune to drug XX. N&E entailing that the XX-mutation is part of our genetics thus makes Pr(R|N&E) low, and I thus come to believe Pr(R|N&E) is low. I believe some time after it’s discovered that drug XX entered our bloodstream, credible scientists have run cognitive tests to confirm that we have the blocking gene. But since this belief came long after drug XX entered my bloodstream, it seems that, like scenario (S4), this would-be evidence for my cognitive reliability (my memory of the cognitive tests) is undermined by drug XX. My belief
I have the XX-mutation defeats R for me here just as it does in scenario (S4).
Scenario (S6): The Probability Thesis is true and Pr(R|N&E) is low, but I do not initially believe this and instead think I am the product of a sort of evolution that makes my cognitive reliability very likely. Later however I study philosophy and see for myself that the probability of my humanoid cognitive faculties being reliable given that I am a product of naturalistic evolution is low. Afterwards I come to believe I have taken an extensive battery of tests that establish my cognitive reliability, but since this belief came long after naturalistic evolution created my cognitive faculties and I believe that given N&E, naturalistic evolution has a high probability of giving me unreliable cognitive faculties (by which I mean I believe that Pr(R|N&E) is low and naturalistic evolution is what gives me my cognitive faculties), it seems that this would-be evidence for my cognitive reliability (my memory of passing the cognitive tests) is undermined by the effects of naturalistic evolution similar to how naturalistic evolution giving me the XX-mutation in scenario (S5) undermines my would-be evidence for R, and so it seems that I have a defeater for my belief that my cognitive faculties are reliable.
To strengthen the case further, let XX symbolize “drug XX entered one’s bloodstream” and let’s stipulate that in scenarios (S1) through (S5), Pr(R|XX) (the probability of R given that XX entered one’s bloodstream) is as low as Pr(R|N&E) is in scenario (S6). So above we have a slippery slope of scenarios. The idea is that if R is defeated in (S1), then it is defeated in (S2), and if R is defeated in (S3), then it is defeated in (S4), and so forth. Or to put it more explicitly in the form of a deductively
valid argument where lines 2 through 6 are
material conditionals (a material conditional is where “If
P, then
Q” just means “It’s not the case that
P is true and
Q is false”):
- R is defeated in scenario (S1).
- If R is defeated in scenario (S1), then R is defeated in scenario (S2).
- If R is defeated in scenario (S2), then R is defeated in scenario (S3).
- If R is defeated in scenario (S3), then R is defeated in scenario (S4).
- If R is defeated in scenario (S4), then R is defeated in scenario (S5).
- If R is defeated in scenario (S5), then R is defeated in scenario (S6).
- If R is defeated in scenario (S6), then the Defeater Thesis is true.
- Therefore, the Defeater Thesis is true.
Because premises (2) through (7) are material conditionals, the only way e.g. premise (2) could be false is if R is defeated in (S1) and R is not defeated in (S2). So identifying a false premise would identify where the slippery slope stops. But it’s difficult to find a premise that is plausibly false. It would clearly be irrational for me to believe that Sam’s cognitive faculties are reliable given the information I have in scenario (S1), in which case premise (1) is justifiably true. There also doesn’t seem to be a relevant difference between scenarios (S1) and (S2) whereby R is defeated in (S1) but not (S2), and the same goes for every pair of scenarios in premises (2) through (6): there doesn’t appear to be a relevant difference between the two scenarios in any individual premise where R is defeated in one but not the other. So each premise (2) through (6) would seem to be justifiably true.
What about premise (7)? Thinking that R is defeated in scenario (S6) but not for the general person who accepts the Probability Thesis seems especially implausible. Scenario (S6) is where I see the Probability Thesis is true and I believe I have passed a battery of cognitive tests that confirm my cognitive reliability. If R is defeated even here, it’s hard to imagine what relevant difference there might be between this and another person who sees that the Probability Thesis is true. Part of the point of (S6), after all, is that any would-be evidence for R would be undermined just as any would-be evidence for R is undermined in scenario (S5). It seems that for the Defeater Thesis to be false, R would have to be undefeated in scenario (S6).
If however R is not defeated in (S6), where does the slippery slope stop and why? Where does there exist a relevant difference between two scenarios that saves R from defeat? It’s particularly hard to find a relevant difference between (S5) and (S6). One might say in (S6) we know of overwhelming evidence in addition to N&E that makes R likely, whereas that’s not the case in (S5). But why exactly do we have this evidence in (S6) but not in (S5)? To make the problem for this more explicit, imagine that the two worlds of (S5) and (S6) are essentially identical apart from the differences entailed in (S5), such that I believe that the specific type of naturalistic evolution my species is a product of has given me genes that (together with proper nutrition etc.) makes it likely that my cognitive faculties are reliable, that cognitive science and evolutionary biology has given us strong evidence for human cognitive reliability, that truth-conducive faculties are adaptive in Earth primates, and so forth. I also believe that we have overwhelming scientific evidence that we have the blocking gene to nullify the effects of the XX-mutation. Yet all this alleged evidence for cognitive reliability seems undermined when one accepts the evidence long after drug XX entered the bloodstream. So how exactly is it that the alleged evidence for R is undermined in scenario (S5) but not in scenario (S6)? If there is a relevant difference between the two scenarios, what is it?
One could believe that the relevant difference between scenarios (S5) and (S6) is N&E’s mechanism of probable cognitive unreliability (MoPCU), i.e. whatever it is that makes Pr(R|N&E) low. In scenario (S5), that mechanism is the XX-mutation; whereas in scenario (S6) N&E’s MoPCU is (presumably) something else, e.g. perhaps what makes Pr(R|N&E) low in (S6) is semantic content being causally irrelevant and invisible to natural selection coupled with the fact that the enormous variety of “garbage” belief sets akin to dreams vastly outnumber belief sets that accurately resemble one’s external reality. So, one objection to the argument is that N&E’s different MoPCU is the relevant difference between (S5) and (S6) such that R is defeated in (S5) but not in (S6), which would make premise (6) false.
But there’s a problem with N&E’s MoPCU being a relevant difference. N&E’s MoPCU in scenario (S6) is functionally identical to drug XX in that it induces the
same type of cognitive unreliability with the
same probability (in the sense that Pr(R|XX) = Pr(R|N&E)). Thus, which mechanism is N&E’s MoPCU does not seem to be a relevant difference. Apart from N&E’s MoPCU, there doesn’t appear to be any other plausible relevant difference between (S5) and (S6) here. If so, then the would-be evidence for cognitive reliability in scenario (S6) is undermined just as it is in scenario (S5). In (S5), N&E’s MoPCU is the XX-mutation; in (S6) the mechanism is different, but it’s functionally identical to drug XX (inducing the same type of cognitive unreliability with the same probability). It therefore seems arbitrary to hold that R is defeated in (S5) but not in (S6).
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[4] Well, not
the analogy, since Alvin Plantinga himself has several variants, and I’m using my own variety here. Still, drug XX rendering one’s cognitive faculties unreliable for some portion of those who ingest it is common in all of Plantinga’s renditions of this analogy that I’ve read.