Not a brain, but a brain in a vat. That's what won't die, at least in that part of the world (if there is one!) inhabited by philosophers. "An unspeakable horror from hell!" Indeed.
It's difficult to imagine a less serviceable avenue of thought than that indulged in by many philosophers for many centuries, relentlessly wondering if there is what is suggestively referred to as an "external world" and, if so, if it can be known by us. Worse, though, is their shameless tendency to write on the subject, if indeed it can be called one. The consideration of the "question" is singularly purposeless, not merely because it's incapable of resolution, not indeed merely because it's questionable there is a "question" to be addressed, but because its consideration along with any possible resolution makes no difference to us, or to our lives.
It can't be disputed that we always act as if there is an "external world" and for the most part that our belief in it and its characteristics is substantially justified. Anyone who seriously questions it could be accused of deliberately and thoughtlessly disregarding his purported belief. Such a person would have to be mentally ill--"There is no world, but I live in it" isn't a statement most would make, and if one would make it seriously one would be treated, justly, as not quite sane. That is what's done by philosophers who chew on this old chestnut and do so verbally and in print, to the amazement and bewilderment of those who listen to them or read their words (assuming, of course, that they exist).
What's particularly curious about this fixation with our claimed ignorance is that it's sometimes asserted that our incapacity is established by "scientific evidence." There is of course the problem that it's questionable whether there can be any evidence, scientific or otherwise, if we can't know the "external world." But it would seem obvious that the scientific evidence indicates that we evolved over time through our interaction with the rest of the world and interact with it consistent with our expectations time and time again. If the results of experiments are "scientific evidence" it's apparent that scientific evidence in support of the existence of an "external world" is overwhelming.
Why, then, is this fantastic subject one of continuing interest and effort? Let's ask, following Cicero: Qui bono fuisset? Where we humans are concerned, it's always important to consider who benefits from a particular act, question or issue. Benefit--ours in particular--is always uppermost in our minds.
Who would benefit from a belief that we can't know the world, that we're separate from it, that it isn't real, or less than real; isn't true or less than true?
Those who thrive on it, clearly. Philosophers, of course, do so. For some it's their bread and butter. Most if not all of the religious, as well. "My kingdom is not of this world"--so said Jesus or so said someone who said Jesus said it. The clergy in particular. What would they do if people didn't think there was a world beyond this one; a world better than this one, which is the "real world"?
Anyone who benefits from disregard of or disparagement of the world as it is conceivably gains from the perception that there's something fundamentally wrong with things, so we may have to include politicians and pundits as well among those who claim the world isn't as it appears to us.
No wonder that Plato, who first wrote extensively of a reality not of this world, was a totalitarian at heart. If the world isn't real, then clearly we should seek what is real. If we can't, we should be made to do so, for our own good.
And so we see what metaphysics and epistemology can lead to, if untrammeled by the world! Without weights or guide lines, they ascend uncontrolled into the air like the balloon of the Wizard of Oz, they render us inhabitants of that city in the sky, Cloud Cuckoo Land of Aristophanes. No wonder, perhaps, that Heidegger was a Nazi.
