All truth is conditional, all knowledge mutable. It doesn't take a lot of reflection to arrive at these simple, yet significant, conclusions. Where you go from there is more complex.
Consider truth. I'll use an example inspired by a movie I saw recently. We can say it is a "truth" that humans have the capacity to directly inflict on one another suffering so monstrous as to defy description—but (the condition) in reality that capacity isn't realized except in unusual situations, e.g. extreme external social pressures or extreme internal mental disorder. We can say it is a "truth" that death is inevitable, but that too may only be because medical science hasn't advanced enough. Already, multiple research avenues toward medical immortality are beginning to yield fruit, and that doesn't even acknowledge the possibility of purely electronic means of death avoidance (brain uploading).
Now consider knowledge. Any day now, physicists or astronomers could make a discovery that overturns much of what we hypothesize about the universe, or even of the history of our own planet. Or, on the more mundane scale, that close friend or dear relative who you "know" could never do such a thing? You really never know. (As others will tell you after the fact.)
This is perhaps the key reason why so many relationships break down even when several ingredients for success are present. Those of us who build identities around what we take to be truths or certainty, external to ourselves, are setting ourselves up for disappointment, potentially on a crippling scale.
One confronted by such realizations naturally tries to arrive at some kind of fundamental, unchangeable truth and/or knowledge to which they can cling. A deity or a sports team may provide enough foundation for some, beyond which they choose not to explore. Others may breathe a sigh of existential relief upon deciding "I think, therefore I am"—yet even this is conditional and mutable.
We cannot "know" as a foundational "truth" that we are real, and not some form of elaborate simulation. Maybe you've heard this one: Swedish philosopher Nick Bostrom argues that
- "at least one of the following propositions is true: (1) the human species is very likely to go extinct before reaching a 'posthuman' stage; (2) any posthuman civilization is extremely unlikely to run a significant number of simulations of their evolutionary history (or variations thereof); (3) we are almost certainly living in a computer simulation. It follows that the belief that there is a significant chance that we will one day become posthumans who run ancestor-simulations is false, unless we are currently living in a simulation."
I think proposition 2 is the one to be considered implausible in Bostrom's formulation, leaving us with 1 (which we might as well not discuss here) and 3. His reasoning can be found at the link provided above, but it's not as far-fetched as it seems at first glance. A civilization with the technology to do so might have any number of reasons to simulate evolutionary history, from anthropology to marketing research to recreation. Yesterday I learned that it's possible to run a simulation (in the form of a dream) within the video game The Sims 3. You can also already specify which general genres of video game your sims play when they feel like wasting time (emulating their creator). Perhaps in The Sims 4 you will be able to track, or even control, your sims' sims (which, for clarity, we would have to refer to as simsims).
Now obviously, The Sims is a computational resource hog even now, but I expect Bostrom presupposes (and I would agree) that philosophical debates such as this must not be constrained by what technology can do now, or even what we might guess it will do. Even the currently-known potential of quantum computing seems to suggest that it could be possible to simulate a planet full of monkey-people, right down to the workings of their cells, and the cells of everything else on and near that planet. So we might be the sims of some other entities that created our reality, observe our activities, control our happenstances, or simply forgot about us and left the program running. Such a notion is bound to depress some people, especially if they don't consider sims "people"—and right now we have no means of proving this notion false.
But the good news is that we have equally zero means of being certain that we aren't what we think we are. In that light, the only reasonable thing is to assume that we aren't simulations until confronted with incontrovertible evidence to the contrary—to adopt a sort of existential agnosticism. "I think, therefore I am, though I may not be what I think I am."
The opposite of this reasonable approach is solipsism, or as it could be less charitably termed, abandonment of reality. British physicist David Deutsch has written much more, and much better than I could, on this and related topics. If you got this far in this post, you might want to check out his books The Fabric of Reality and The Beginning of Infinity.
P.S.: Sorry about the Neil Diamond earworm.