I find myself, right now, 99% confident that I am who I think I am, living in a broad world of the kind I think I live in. The remaining 1% of my credence I reserve for all radically skeptical scenarios combined.
Most of us, I think, don't reserve even 1% of our credence for radically skeptical scenarios. Maybe if you're philosophically inclined and not entirely hostile to skepticism, you'd be willing to say, in certain reflective moments, that there is some chance, maybe about 1%, that some radically skeptical scenario obtains. But such acknowledgements are typically not truly felt and lived -- not in any durable way. Skeptical doubts stay in the classroom, in the office, in the books. They don't come home with you.
What if skeptical doubt did come home with you? What would it be like really to live with 1% of your credence distributed among radically skeptical scenarios?
It depends in part on what the scenarios are. Let mention three that I have trouble entirely dispelling.
Random origins skepticism. Most physicists think that there is a finite though tiny chance that a brain or brain and body could randomly congeal from disorganized matter. In a sufficiently large and diverse universe, we should expect this chance sometimes to be realized. A question that then arises is: Are randomly congealed beings relatively more or less common than beings arising from what we think of as the ordinary process of billions of years of biological evolution? A difficult cosmological question! I see room for some doubt about the matter, so it seems I ought to reserve some subjective credence for the possibility that such freak-chance beings are common enough that I might be one of them, and thus that I lack the sort of past, and probably future, that I think I have. (This is the Boltzmann brain hypothesis.)
Simulation skepticism. If it is possible to create consciousness artificially inside computers, then likely it is also possible to create conscious beings with radically false autobiographical memories and radically false impressions about the sort of world they live in. I don't feel I can entirely exclude the possibility that I am such a radically mistaken artificial being -- for example a being limited to a few hours' existence in a 22nd-century child's computer game. (This is a skeptical version of the "simulation" possibility, discussed non-skeptically by Nick Bostrom here and by David Chalmers here.)
Dream skepticism. Zhuangzi dreamed he was a butterfly. After he woke he wasn't sure if he was Zhuangzi who had dreamed he was a butterfly or a butterfly dreaming he was Zhuangzi. I'm inclined to think the phenomenology of dreaming is very different from the phenomenology of waking, and thus that my current experiences are excellent grounds for thinking I am indeed awake. But I am not absolutely sure of that. And if I might be dreaming, then I might not really have the family and career and past life I think I do.
I don't believe that any such skeptical scenario is true, or even very likely. On reflection, I am inclined to grant all such skeptical scenarios combined about 1% of my credence. In a way, that's not much. But in another way, that's quite a bit.
Suppose someone came to me with two ten-sided dice and said this: I will roll these two dice, and if they both come up "1", you will die. Would I rest in gentle confidence that a 99% chance of living is quite an excellent chance? Or suppose I were to learn that seven unnamed students from my daughter's largish elementary school had just been abducted into irrecoverable slavery. Would I feel sorry for those students but feel no real concern about my daughter, since the odds are so good she is not among them?
I sit on by back patio. If some radically skeptical scenario is true, then my daughter does not exist, or I will be dead within a minute as the disorganized soup from which I've congealed consumes me again, or my life will end in an hour when the child grows bored with the game in which I am instantiated. Should I be unconcerned about these possibilities because I judge it to be 99% likely that nothing of this sort is so?
I hear a voice from inside the house. It's my wife. How would it change things if instead of taking it for granted that she exists, I held it to be 99% likely that she exists? -- or 99.5% likely, if I allow a 50% chance of her real existence given a skeptical scenario?
Often, I visually imagine negative events that have a small chance of occurring. This morning, the newspaper told me of five teenagers who died in a street race; I looked at my own teenage son, who was readying for school, imagining his death. Or I'm passenger in an airplane descending through heavy fog and turbulence, and I imagine a crash. I hear of a streak of identity thefts in Riverside, see that someone has messed with our mailbox, and imagine our bank accounts drained. If I am a 1% skeptic, shall I then also imagine the world's suddenly splitting into void, Godzilla's rising over the horizon for the child's entertainment, my suddenly floating off into the air, opening my front door to find no ordinary suburban street but rather Wonderland or darkness?
I have, in fact, started to imagine these things more often. I don't believe them -- no more than I believe the plane will crash. Given non-skepticism, the plane is much less than 1% likely to crash. If I am a 1% skeptic, then I should probably think it more likely that am I Boltzmann brain or a short-lived artificial consciousness or a much-deceived dreamer, than that the plane will crash. Would it be more rational, then, for me to dwell on those possibilities than on the possible plane crash?
I am undecided about doing some chore. I could weed the yard. Or I could sip tea, enjoy the shade, and read Borges. I teeter right on the cusp about which is the wiser choice. But skepticism has not yet crossed my mind. Once it does, the scale is tipped. The 1% chance that the weeds are an illusion or a mere temporary thing -- the small but non-trivial possibility that this moment here is all I have before I die or the world collapses around me or I wake to something entirely new -- favors Borges.
If my credence in the durable reality of this patio and this book were to fall much below 99% -- if my credence were to fall, say, to only 80% or 50%, I would be laid low. My death might well be upon me within minutes. I would seek my family, my seeming-family; at the same time, my doubts would isolate me from them. I could not, I think, feel full proper intimacy while I regard my partner in intimacy as fairly likely to be mere froth or illusion. Even if my wife and children are not froth and illusion, such large doubt is disaster, for it would derange my choices and emotions, and few people would understand my doubts, even if my own careful reflection revealed those doubts to be well-grounded. Everyone around would see me only as insane with foolish philosophy.
1% skepticism does not have this effect though. I can still enjoy my Borges, even flatter myself somewhat with my excellent excuse for avoiding the weeds, which I am of course in fact almost certain do exist.
[For related reflections see Waterfall Skepticism.]