A "freak observer" or "Boltzmann brain" is a conscious being who did not arise in the normal way on a large, stable planet, but who instead congealed by freak chance out of chaos, due to a low-probability quantum or thermodynamic fluctuation -- a conscious being with rich seemingly sensory experience, rich seeming-memories, and capable of sophisticated thoughts or seeming-thoughts about itself and its position in the universe. By hypothesis, such a being is massively deluded about its past. And since random fluctuations are much likelier to create a relatively small system than a relatively large system, and since a relatively small system (such as a bare brain) amid chaos is doomed to a short existence, most freak observers will swiftly perish.
If certain cosmological theories are true, then almost all conscious systems are freak observers of this sort. Here's one such theory: There is exactly one universe which began with a unique Bang, which contains a finite number of ordinary non-freak observers, and which will eventually become thin chaos, enduring infinitely thereafter in a disorganized state. In any spacetime region there is a miniscule but finite chance of the spontaneous freak formation of any finite organized system, with smaller and less organized systems vastly more likely than larger and more organized systems. Given infinite time, the number of spontaneously formed freak observers will eventually vastly outnumber the normal observers. Whatever specific experiences and evidence I take myself now to have, according to this theory, to any finite degree of precision, there will be an infinite number of randomly generated Eric Schwitzgebel clones who have the same experiences and apparent evidence.
Can I prove that I am not a freak observer by counting "1, 2, 3, still here"? Seemingly no, for two reasons: (1.) By the time I reach "still here" I am relying on my memory of the "1, 2, 3", and the theory says that there will be an infinite number of freak observers with exactly that false memory. (2.) Even if assume knowledge of my continued existence for three seconds, there will be an infinite number of somewhat larger freak observers who congealed simultaneously with a large enough hunk of environment to exist for three seconds, doing that apparent count. If I am such a one, I will very likely perish soon, but it is not guaranteed that I will perish, and if I don't perish and thus conclude that I am not a freak I have ignored the overwhelming base rate of freaks to normal observers.
Suppose that given the physical evidence such a cosmology seems plausible, or some other cosmology in which freak observers vastly outnumber normal observers. Should I conclude I am probably a freak observer? It would be a strange conclusion to draw!
One interesting argument against this conclusion is the cognitive instability argument (Carroll 2010; Davenport & Olum 2010; Crawford 2013): Suppose that my grounds for believing that I am a freak observer are Physical Theory X, which I accept only conditionally upon believing that I have good empirical evidence for Physical Theory X. If I am a freak observer, then, contrary to the initial assumption, I do not have good empirical evidence for Physical Theory X. I have not, for example, despite my contrary impression, actually read any articles about X. If I seem to have good empirical evidence for Physical Theory X, I know already that that evidence is almost certainly misleading or wrongly interpreted -- either I do have the properly-caused body of evidence that I think I have, that is, I am not a freak, and that evidence is misleadingly pointing me to the wrong conclusion about my situation; or I am a freak and I don't have such a body of properly-caused evidence at all.
For this reason, I think it would be irrational to accept a cosmological theory that implies that almost all observers are freak observers and then conclude that therefore I am also a freak observer.
But a lower-confidence conclusion seems to be more cognitively stable. Suppose our best cosmological theory implies that 1% of observers are freaks. I might then accept that there is a non-trivial chance that I am one of the freaks. After all, my best understanding of the universe implies that there are such freaks, and I see no compelling reason to suppose that I couldn't be one of them.
Alternatively, maybe my best evidence should leave me undecided among lots of cosmologies, in some of which I'm a freak and in others of which I'm not. The possibility that I'm a freak undercuts my confidence in the evidence I seem to have for any specific cosmology, but that only adds to my indecision among the possibilities; it doesn't seem to compel elimination of the possibility that I am a freak.
Here's another way to think about it: As I sit here in my office, or seem to, and think about the scope of the cosmos, I find myself inclined to ascribe a non-trivial credence to some sort of very large or infinite cosmology, and also a non-trivial credence to the hypothesis that given enough time freak observers will spontaneously form, and also a non-trivial credence to the possibility that the freaks aren't vastly outnumbered by the normal observers. If I accept this conjunction of views, then it seems to me that I should also assign a bit of credence to the possibility that I am one of the freaks. To do otherwise would seem to commit me to near certainty on some proposition, such as about the relative nucleation rates of freaks vs. environments containing normal observers, that I wouldn't normally think of as something I know with near certainty.
Or maybe I should just take it as an absolutely certain "framework" assumption that I do have the kind of past I think I have, regardless of how many Eric-Schwitzgebelesque freaks the cosmos may contain? I can see how that might be a reasonable stance. But that approach has a dogmatic air that I find foreign.
If I allow that I'm not absolutely 100.0000000000000000000000000000% certain that I'm not a spontaneously formed freak observer, what sort of credence should I assign to the possibility that I am a freak? One in million? One in ten trillion? One in 10^100? I would like to go low! But I'm not sure that it's reasonable for me to go so low, once the possibility occurs to me and I start to consider my reasons pro and con. I'm inclined to think it is vastly less likely that I am a freak observer than that this ticket will win the one-in-ten-million Lotto jackpot -- but given the dubiety of cosmological theories and my inability to really assess them, should I perhaps be considerably less confident than that about my non-freakish position in the cosmos?