The problem of the external world is a classic topic in epistemology (the theory of knowledge). This challenge, which was first presented by ancient skeptics, is met by proving that what I seem to be experiencing is real. As an example, it would require proving that the computer I seem to be typing this on exists outside of my mind.
Some early sceptics created the problem by noting that what seems real could be a dream. Descartes added a new element by considering that an evil demon might be causing him to have experiences of a world that does not exist. While the evil demon was said to be devoted to deception, little is said about its motive. After Descartes there was a move from supernatural to technological deceivers: the classic brain-in-a-vat scenarios that are precursors to the more recent notion of virtual reality. In these philosophical scenarios little is said about the motivation or purpose of the deceit, beyond the desire to epistemically mess with someone. Movies and TV shows sometimes explore motives of deceit. The Matrix trilogy, for example, presents a backstory for the Matrix. While considering the motivation behind the alleged deceit might not bear on the epistemic problem, it is an interesting subject.
One way to discern a possible motivation for the deceit is to consider the nature of the experienced world. As various philosophers, such as David Hume, have laid out in their formulations of the problem of evil (the challenge of reconciling God’s perfection with evil) the world is an awful place. As Hume has noted, it is infested with disease, suffused with suffering, and awash in annoying things. While there are some positive things, there is an overabundance of bad, thus indicating that whatever lies behind appearances is either not benign or not very competent. This, of course, assumes some purpose behind the deceit. But perhaps there is deceit without a deceiver and there is no malice. This would make the unreal like what atheists claim about the allegedly real: it is purposeless. However, deceit (like design) seems to suggest an intentional agent and this implies a purpose. This purpose, if there is one, must be consistent with the apparent awfulness of the world.
One approach is to follow Descartes and go with a malicious supernatural deceiver. This being might be acting from malice to inflict both deceit and suffering. Or it might be acting as an agent of punishment for my past transgressions. The supernatural hypothesis does have some problems, one being that it involves postulating a supernatural entity. Following Occam’s Razor, if I do not need to postulate a supernatural being, then I should not do so.
Another possibility is that I am in a technological unreal world. In terms of motives consistent with the nature of the world, there are numerous alternatives. One is punishment for some crime or transgression. A problem with this hypothesis is that I have no recollection of a crime or indication that I am serving a sentence. But it is easy to imagine a system of justice that does not inform prisoners of their crimes during the punishment and that someday I will awaken in the real world, having served my virtual time. It is also easy to imagine that this is merely a system of torment, not a system of punishment. There could be endless speculation about the motives behind such torment. For example, it could be an act of revenge or simple madness. Or even a complete accident. There could be other people here with me; but I have no way of solving the problem of other minds, no way of knowing if those I encounter are fellow prisoners or mere empty constructs. This ignorance does seem to ground a moral approach: since they could be fellow prisoners, I should treat them as such.
A second possibility is that the world is an experiment or simulation of an awful world, and I am a construct within this world. Perhaps those conducting it have no idea the inhabitants are suffering, perhaps they do not care. Or perhaps the suffering is the experiment. I might even be a researcher, trapped in my own experiment. Given how scientists in the allegedly real world have treated subjects, the idea that this is a simulation of suffering has considerable appeal.
A third possibility is that the world is a game or educational system of some sort. Perhaps I am playing a very lame game of Assessment & Income Tax; perhaps I am in a simulation learning to develop character in the face of an awful world; or perhaps I am just part of the game someone else is playing. All of these are consistent with how the world seems to be.
It is also worth considering the possibility of solipsism: that I am the only being that exists. It could be countered that if I were creating the world, it would be much better for me and far more awesome. After all, I write adventures for games and can imagine a far more enjoyable world. The easy and obvious counter is to point out that when I dream (or, more accurately have nightmares), I experience unpleasant things on a regular basis and have little control. Since my dreams presumably come from me and are often awful, it makes perfect sense that if the world came from me, it would be comparable in its awfulness. The waking world would be more vivid and consistent because I am awake, the dream world less so because of mental fatigue. In this case, I am my own demon.