Stephen Law's Evil God Challenge (EGC) is quite simple, and complex, at the same time (like some god concepts!). There have been many objections raised against it, but I think they mostly miss the very simple crux, which is, I think:
If they did not rule out an evil god for some other reason, do believers think that the evidence supports its existence?
(It's well worth reading the entire paper, linked above, to get the full flavour)
It's clear that some theists do understand the problems this poses for them, since much wriggling can be observed, amid cries of foul play. Now, plainly many do rule out the evil god for some other reason; some, no doubt, for long considered reasons. However, given how superficially many come to their god belief, through childhood indoctrination, for example, there's also a suspicion that some have simply not thought much about the possibility of an evil god, that it's simply not an idea that they would entertain, that it's evident that it does not exist. If you're a believer who thinks the idea of an evil god is obviously ruled out by one's experiences, then you have a serious problem with the EGC. You would need to show a significant asymmetry between the evidence for the good god and the evil one, and, whilst there is some asymmetry, I just don't think it's significant enough.
William Lane Craig, for example, can see that all his arguments for god, but one, are equally valid for an evil god as for a good god, so his entire belief in a good god rests on the very dubious argument from morality. He sees the danger from evidence inherent in the EGC (that the evidence rules out his god), so adopts a radical scepticism to avoid that disturbing truth.
Thomists, on the other hand, and as I understand it, have a notion of 'being' and 'good' that makes an evil god a contradiction in terms, effectively. That's fine, and if they really believe in these notions and definitions, there's nothing, in logic, to stop them. But there are any number of logical possibilities that fit with the world in which we live, and simple logical possibility seems a very low standard by which to judge one's belief. Why wouldn't one want to compare it with the evidential data to see how well it fits, compared to other scenarios, for example? After all, the only reason anyone believes in a god in the first place is because of the situation in which they find themselves, so their belief would not exist without the initial contemplation of evidence.
Law suggests, then, that the EGC can still be run, even though a concept is paradoxical. In other words, we say, OK, the evil god is nonsensical, but run the challenge anyway as if it were not. There can be no doubt that even Thomists would have to agree that the evidence rules out the evil god, and, therefore, so it must rule out the good god (unless they adopt a Craig-style radical scepticism). Thomists protest that the evidence is irrelevant because of their concept of god. Would Thomists accept a non-believer simply dismissing any discussion of the evidence for god because she found the concept incoherent? I don't know. So, is Law's a valid move, to run the EGC against an impossible concept?
I'm not entirely sure. It's right that there is little point in assessing the evidence for married bachelors or invisible pink unicorns, to give two more examples of impossible concepts - we can rule them out a priori. But does this mean that we cannot consider what evidence there might be if, defying logic, they did exist, via a thought experiment? Can we still identify attributes of these paradoxical beings and consider how a world would look with them in? We can look for evidence of bachelordom, and marriage certificates, so we could look for evidence of each attribute - a world with no marriage certificates would rule out anyone being married, for example. So it's tempting to consider this a valid move in the challenge. However, if one considers the evidence for the paradox in toto, I'm less sure. What evidence could there be for (married bachelors)? Well, none, I would think, because they cannot exist.
So, if we consider what a world would look like with a paradoxical evil god in it, and one without, there would be no difference between the two, because the concept cannot exist in either; this suggests the EGC does not apply. However, if we consider what a world would look like with an evil something, and then, separately, what it would look like with an all-powerful being, of unknown goodness, we would expect different worlds. This suggests the EGC does apply.
On balance, I'm inclined to think it can still be applied, if there are identifiable attributes, which there are in the case of the evil god. We are entitled to look at the state of the world and see if it comports with an evil something; and if that something is an extremely powerful being, we can expect certain consequences to flow from that. So, if this can be agreed, the EGC will apply even to incoherent god concepts.
Ultimately, of course, the Thomist god is not a concept worth entertaining because there is no evidence offered to support it among the infinite logically possible beings that might exist in our universe. Only a credulous mind could possibly commit to it, rather than simply note its logical possibility.
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