A scientific response to the Brain in a Vat

Scientia Salon is an enjoyable webzine discussing philosophical matters, which recently addressed an old conundrum: how do we know we are not a brain in a vat? As I see it, this question is straightforwardly answered by the usual scientific method, so here I’ll summarise the argument that I advanced in the Scientia Salon discussion.

The Matrix-style scenario, which dates back to the skepticism of Descartes, supposes that we are a brain kept alive in a vat, being fed with a stream of inputs generated by an Evil Genius. Everything that we experience as sense data is not real, but is artificially simulated and fed to us. Since, ex hypothesi, our stream of experiences is identical to that in the “real world” explanation, we cannot know for sure whether or not we are such a brain in a vat.

How to respond? First, the whole point of science is to make sense of our “stream of experiences”. We do that by looking for regularities and patterns in those experiences, and we develop those into descriptions and explanations of the world (I’ll use the term “world” here for the sum of those experiences, regardless of whether they derive from our contact with a real world, or from a simulated world being fed to us).

Science attempts to build the best models of that world, meaning the most parsimonious explanations with the greatest explanatory and predictive power. The more parsimonious the description and the greater the predictive power the more likely the model is to be close to the true state of affairs. I’ve previously written a post justifying parsimony, but the main point is that the set of all true statements is vastly smaller than the set of all possible statements, and thus we are only likely to alight on a true statement when guided to it by evidence. Any statement not supported by evidence is most likely to be wrong and thus should be discarded. Thus science’s task is finding the model with the least information content that adequately describes the world.

Now suppose that we are a brain in a vat. Science’s task is still to describe and model everything, including the “stream of experiences”. Regardless of whether the “world” is real or simulated, the standard scientific “real-world model” gives the greatest parsimony and predictive power in describing that world. Any departure from that standard model would result in a worse account (one that is less parsimonious and less predictive about that stream of experiences). That’s because science’s models do work very well about our world and are the best that we have.

One might object that in a “brain in vat” scenario the Evil Genius could feed us any stream of experience he liked, with no rhyme or reason to it, no regularities and no predictability. That is indeed possible, but then the stream of experiences could only be described essentially as a streamed video tape, which is the most information-hungry type of model and one that would be totally useless at predicting anything.

In contrast the “real world” model is very compact in that all one needs are the basic laws of physics and all else follows from that. This model has a vast amount of predictive power, as we know from the fact that engineering works, and planes we build fly, and predictions we make for solar eclipses come true.

The brain-in-vat model has no capacity to make any predictions at all about the stream of experiences, unless we make all sorts of assumptions about the Evil Genius and why he is feeding us the stream of experiences. Yet, ex hypothesi, we can never have any information about the Evil Genius or his doings.

The only way of regaining any degree of parsimony or predictive power is to build the real-world model into the brain-in-vat model. Thus, we can consider three competing models:

(1) The standard scientific real-world model.

(2) A brain in a vat being fed the {standard scientific real-world model}.

(3) A brain in a vat being fed a video-tape stream of information.

The first of these is by far the most parsimonious. The third is the worst, being vastly less parsimonious than the second. The second is worse than the first since it encompasses everything in the first but then adds a wrapper of additional content. Nor is the additional content trivial, since any explanation would have to describe the Evil Genius and the vat and the brain, and give an explanation of where the Genius came from and why he is acting as he is. This will amount to a whole meta-reality, and since that is going to take a quantity of information akin to that of the standard real-world model, the second scenario is going to involve an information content of order double that of the real-world model.

Thus the brain-in-vat model is invoking that vast increase in information content to explain absolutely nothing at all; it is pointed to by literally no evidence whatsoever, since, ex hypothesi, the brain-in-vat scenario envisages that we have exactly the same stream-of-experiences, and thus evidence, that we have in the real-world model. It is thus a hypothesis bad enough to make William of Occam faint with horror, and can be excised by the merest glance towards his razor.

Indeed, this real-world-plus-meta-reality idea is one of a class of models that all fail for the same reason. We can suppose that all sorts of things could be added to our model of the world (parallel dimensions; Narnia-like realities; orbiting teapots à la Russell; Flying Spaghetti Monsters, et cetera) yet if there is no actual evidence for them then the scientific method simply discounts them.

The in-a-vat wrapper to the real-world model is along the same lines, a vast and needless complication that doesn’t in any way improve the model’s fit to evidence. We could just as well imagine any number of other parallel meta-realities that make no difference to what we experience. Since there are an infinite number of such possibilities the chances of any random one of them being actually true is infinitessimal.

Thus, by the adoption of the usual scientific method, invoking Occam’s razor and principles of parsimony and the need for predictive power, we can reject brain-in-a-vat scenarios. That, of course, does not mean that they can be absolutely ruled out, any more than we can rule out any other hypothesis designed to leave no discernible trace at all on our experience of the world (apophatic theologians are particularly good at inventing these), but the chances of any such suggestion being true is too low to merit taking it seriously.