I'm moderately confident that I don't live in a computer simulation.
My reasoning goes like this.
A priori, simulation scenarios are less probable than
non-simulation scenarios.
My evidence is more likely in non-simulation scenarios than in
simulation scenarios.
So: It is highly improbable, given my evidence, that I'm in a
simulation scenario.
By a "simulation scenario", I mean a scenario in which a subject's
experiences of themselves and their environment are generated by a
computer program that simulates an ordinary (non-simulated) subject and
their environment.
I assume that it is a priori possible for a computer program to
generate experiences (and a "subject") by simulating an ordinary subject
with experiences. I'm not 100% sure this is true. (If not, premise 1 can
be strengthened: simulation scenarios have probability 0.) But it seems
plausible, especially if we're liberal about what qualifies as a
computer program and as a simulation.
A popular idea in recent (formal) epistemology is that an externalist conception of evidence is somehow useful, or even required, to block the threat of skepticism. (See, for example, Das (2019), Das (2022), and Lasonen-Aarnio (2015). The trend was started by Williamson (2000).)
An evil scientist might have built a brain in vat that has all the
experiences you currently have. On the basis of your experiences, you
cannot rule out being that brain in a vat. But you can rule out
being that scientist. In fact, being that scientist is
not a skeptical scenario at all. For example, if the scientist in question
suspects that she is a scientist building a brain in a vat, then that
would not constitute a skeptical attitude.
In their contributions to Lewisian Themes, Rae Langton and Jonathan Schaffer both argue that quidditism -- the claim that possible worlds may differ only in which intrinsic properties play which causal/nomological roles -- does not entail skepticism about intrinsic natures because standard replies to skepticism about the external world carry over to skepticism about intrinsic natures.
But it seems to me that there is an important difference: if quidditism is true, we not only lack knowledge about intrinsic natures, but also any beliefs about them.
For some reason, I find Moore's refutation of idealism ("here is a hand; therefore there is an external world") much more convincing than his refutation of skepticism ("I know that here is a hand; therefore I know that I am not a brain in a vat".) Why is that?
In both cases, Moore's argument would not convince his opponent who would obviously reject Moore's premise. So that's not the difference. I think the difference also isn't that skepticism is a philosophically stronger position than idealism. Rather, it seems to me that the premise against idealism is much more certain than the anti-skeptical premise. That here is a hand (or at least that there are hands) is about as certain as non-logical truths get, that I know that here is a hand is not. If I were to compile a list of Moorean facts -- of facts that are at least as certain as any philosophical argument against them --, I would include all kinds of facts about material objects, other people, experiences, mathematics and modality, but knowledge claims probably wouldn't make the list.
I'm always worried when a philosopher claims that it's a virtue of his theory that it rules out certain kinds of scepticism, or when a philosopher criticizes another philosopher (say, a contextualist) for not doing so.
I suppose it would be a good thing if newspapers always told the truth. But what would you say if I offered you a theory on which it is ruled out a priori that something false could be written in a newspaper? That wouldn't be a point in favour of my theory. For it seems intuitively obvious that something false could be written in a newspaper. A theory isn't good just because it entails something which, if true, would be good.
John Hawthorne has some
nice arguments for the view that knowledge is closed under known
implication. I don't know much about knowledge, but it seems to me that
there is a good reason to believe that at least justification -- and hence
presumably also justified true believe -- is not so closed. The reason
is this:
E is some evidence, H and S are alternative and incompatible hypotheses.
(Obvious examples are skeptical scenarios, like E = visual evidence of
a zebra, H = there is a zebra, S = there is a mule disguised as a zebra.) E
strongly supports H: It raises its probability of truth from about 0.3 to
about 0.9. And H implies Not-S. Yet E does not raise the probability of
Not-S. On the contrary, it raises the probability of S.
Let "S(p)" abbreviate "p is strongly supported by the availble evidence".
The picture shows that
S(p) and S(p -> q) does not imply S(q);
S(p & q) does not imply S(p); (let p=-S, q=H)
S(p) does not imply S(p v q); (let p=H, q=-S).
