The brain-in-a-vat scenario is well-known to philosophers and to viewers of The Matrix.
The idea is that you are a disembodied brain suspended in a vat of nutrients, attached to a computer by electrodes. The computer runs a program that produces electrical impulses causing your sensory neurons to fire the same way they would were you really perceiving things.
You can thus have the experience of seeing a tomato without actually seeing a tomato.
If there’s no evidence that one of two competing hypothesis is more likely than the other, it’s irrational to believe either.
For example, if the evidence favors neither of two hypotheses — that the plane crashed because of a mechanical failure or because of pilot error — it’s irrational to believe either.
There’s no evidence that makes either the common-sense worldview of physical objects or the brain-in-vat hypothesis more likely than the other.
Because a person’s experience would be the same.
It’s irrational to believe the common-sense worldview of physical objects.
No one knows whether physical objects exist.
Response Making a Virtue of Necessity
Normally, when a person concludes there’s no rational basis for a belief, they stop believing, e.g. the belief that Iraq had WMDs.
But that’s not what happens when a person, convinced by the Brain-in-a-Vat argument, concludes that belief in the external world is irrational. Skeptics continue believing there’s an external world because, as David Hume pointed out, “nature is always too strong for principle.”
Thomas Reid, however, a contemporary of Hume’s, used the inability to give up the belief in an external world as an argument for its rationality.
“Methinks, therefore, it were better to make a virtue of necessity; and, since we cannot get rid of the vulgar notion and belief of an external world, to reconcile our reason to it as well as we can; for, if Reason should stomach and fret ever so much at this yoke, she cannot throw it off; if she will not be the servant of Common Sense, she must be her slave.”
Reid’s idea was that belief in the common-sense worldview of physical objects is rational because no rational argument can dislodge it. What’s rational to believe must be believable.
“This, after all, you know, really is a finger: there is no doubt about it: I know it, and you all know it. And I think we may safely challenge any philosopher to bring forward any argument in favor either of the proposition that we do not know it, or of the proposition that it is not true, which does not at some point, rest upon some premiss which is, beyond comparison, less certain than is the proposition which it is designed to attack.”
Charles Sanders Peirce
“Belief does not make us act at once, but puts us into such a condition that we shall behave in some certain way, when the occasion arises. Doubt has not the least such active effect, but stimulates us to inquiry until it is destroyed.”
“We are like sailors who have to rebuild their ship on the open sea, without ever being able to dismantle it in dry-dock and reconstruct it from its best components.”
“All reasoning must be from first principles; and for first principles no other reason can be given but this, that, by the constitution of our nature, we are under a necessity of assenting to them.”
Willard Van Orman Quine
We adopt “the fundamental conceptual scheme of science and common sense.”
“But a Pyrrhonian cannot expect, that his philosophy will have any constant influence on the mind: or if it had, that its influence would be beneficial to society. On the contrary, he must acknowledge, if he will acknowledge anything, that all human life must perish, were his principles universally and steadily to prevail. All discourse, all action would immediately cease; and men remain in a total lethargy, till the necessities of nature, unsatisfied, put an end to their miserable existence. It is true; so fatal an event is very little to be dreaded. Nature is always too strong for principle.”