The External World: Preliminary Experimental Evidence of Its Existence

(collaborative with Alan Moore)

Radical solipsism is the view that nothing exists beyond my own stream of conscious experience. I’ve been wondering: Can I show radical solipsism to be false? (Reader, if you exist, then radical solipsism must be false, but – and I hope you’ll forgive me – it doesn’t follow that I can justifiably reject radical solipsism, nor that you can, with the pronoun re-indexed to your own case.)

The two most famous attempts to prove the existence of the external world are Descartes’ and G.E. Moore’s. Descartes’ crucial move was his inference from the existence of the idea of divine perfection to the conclusion that only God could be the source of such a fabulous idea – a highly dubious inference. G.E. Moore argues from “here is one hand” and “here is another” to the conclusion that “there are external things”, but he doesn’t attempt to prove “here is one hand”. Therefore, Moore starts (as he acknowledges) by taking for granted that at least one thing exists beyond his stream of conscious experience.

Thus, neither author convincingly proves the falsity of radical solipsism. Now maybe I shouldn’t need any proof that the external world exists. Perhaps, indeed, as Wittgenstein suggests, it’s a kind of philosophical sickness to want proof. So be it; I’m sick. Still, can I get what my sick mind wants?

Accepting the task at face value, the most promising approach, I think, is abductive inference or inference to the best explanation: The best explanation of why I have this pattern of conscious experiences is that something exists behind my experience that helps shape it. Maybe that’s reason enough to reject radical solipsism, even if it isn’t strict proof. But: Is the existence of some sort of external world really the best explanation of my stream of experience? I’d prefer not to wave my hands vaguely here. I wonder if I can design an experiment or series of experiments to put solipsism to scientific test.

A fair game needs ground rules. The skeptic’s position is obviously unassailable if I must prove every premise of any potential argument. On the other hand, I shouldn’t, like G.E. Moore, invoke premises that already assume the falsity of radical solipsism. For purposes of this study, then, I’ll allow myself tentatively to accept the following:

• introspective knowledge of sensory experience and other happenings in the stream of experience,
• memories of past experiences from the time of the beginning of the experiment (but not before),
• concepts and categories arrived at I-know-not-how and shorn of any presumption of real grounding in the external world,
• the general tools of reason and scientific evaluation to the extent those don’t build in any assumptions about the things beyond the stream of experience.

Accepting these ground rules, most patterns in my stream of experience don’t, I think, demand explanation by anything beyond my stream of experience. Here’s one kind of pattern: When I have an experience as of deciding to close my eyes, I experience a certain sort of dark field (or colorful Eigenlicht). It needn’t follow that I have real external eyes or am being affected by real changes in visual input. This pattern might simply be a pattern within my stream of experience, reflecting some sort of direct causal relationship between one type of experience (as of eyes closing) and another (as of a dark field). The same holds for the patterns I notice as I seem to shift my eyes around or as I seem to look at an object and then grasp it.

Nor does the randomness or uncontrollability of experience demand explanation by appeal to external things. I might have an experience as of tossing a six-sided die, accompanied by a desire for a six and a feeling of not knowing what the outcome will be, followed by an experience as of seeing a four. I might have the experience of opening a closet and feeling surprised by the contents. Such experiences might simply reflect randomness, unpredictability, and uncontrollability within my stream of experience rather than the shaping of experience by forces beyond.

What evidence, then, might I raise against radical solipsism? Here’s my first experiment. I find the experiment somewhat convincing, though I don’t find it entirely convincing – nor do I expect that you will (even if you replicate it yourself, and assuming you exist). As often in the sciences, follow-up experiments will be required to help undermine alternative possible explanations of the results.

First, I do something that I experience as like opening Microsoft Excel. Next, I do something that seems like programming Excel to generate random numbers between 1000 and 3999, excluding numbers divisible by 2 or 5. Next, I do something that seems like programming Excel to determine whether these numbers are prime, but without actually dragging down the formula to execute the calculation. Next, I briefly examine each number, one by one, and try to guess whether it’s prime. To conclude the experiment, here’s what I will do next (though I haven’t yet done it): I will do what seems like executing the calculation in Excel, then compare my guesses with the apparent Excel results, and then confirm the apparent Excel results by hand calculation.

On the assumption that radical solipsism is true, it’s reasonable to predict that either my guesses will match the apparent Excel outputs or my hand calculations will fail to confirm the accuracy of the apparent Excel outputs. I make this prediction because, if radical solipsism is true and the only thing that exists in the universe is my own stream of conscious experience, there will be no device whose calculating capacity outruns my own calculating capacity (except insofar as that calculating capacity is implicit in the laws governing direct relationships among my experiences – an important caveat that I will address in a minute).

I have generated 20 four-digit numbers as described above. Among those 20 numbers, there are 5 that I have guessed are prime. I then execute the apparent spreadsheet calculation. Excel appears to tell me that 4 of the 20 are prime, only one of which is among the 5 that I had guessed to be prime. Finally, I calculate by hand and confirm the apparent Excel results in every case, using my own mathematical reasoning. Thus, I now know that 13/20 of my guesses were correct, compared to 20/20 from the apparent Excel output. This difference in ratio is statistically significant by Fisher’s exact test (hand calculated), with a one-tailed probability of p = .004. Therefore, I conclude that there exists something in the universe with an ability to mark numbers as prime vs. nonprime that exceeds my own ability to do so, at least insofar as I am constituted wholly by my stream of conscious experience. Radical solipsism appears to be scientifically disconfirmed.

Two caveats:

First, I don’t know what that external thing is. It appears to be a Microsoft Excel program implemented on a laptop computer, but I don’t regard myself as having established that fact. The calculation might even have been executed by a nonconscious part of my own mind, in which case some weak form of solipsism might still be true.

Second, there are, of course – as there always are in scientific research – possible alternative explanations of the results. The ground rules of the current exercise require that I consider these explanations using ordinary scientific standards insofar as those standards can be implemented while bracketing the question of whether an external world exists. Some alternative explanations I hope to rule out in future experiments. But at least one seems worth mentioning right away: I have already granted that there may be laws that relate experiences to each other without implying the existence of anything beyond experience. These laws might in fact have a level of organization beyond what I can consciously appreciate as they unfold. For example, the laws relating the colors of apparent visual stimuli with the complementary colors of the afterimages produced by those apparent visual stimuli may involve regularities that I can’t immediately appreciate. Could there, then, be some similar law of experience relating the four-digit numbers in the relevant Excel column with the marking as prime vs. nonprime in another column?

That would seem a strange law for the following reason: The image/afterimage law and other similar laws or patterns of experience, such as the eyes-open/eyes-closed pattern and patterns of relationship between visual appearance and tactile appearance, are laws that relate surface features of one experience with surface features of another, e.g., color of image to color of afterimage, smoothness of visual appearance to smoothness of tactile appearance. In contrast, the proposed experiential law governing my apparent Excel outputs concerns a non-superficial feature of my visual experience – the number represented by the numeral I interpret on the basis of the apparent screen pixels. The posited solipsistic law would relate semantic features of one visual experience to semantic features of another, with the first set of features apparently undetectable by me (and thus by any person, if solipsism is true), though the features were subsequently confirmable by me through laborious calculation. To posit the existence of such a weird semantic law relating two sensory experiences of apparent Excel columns seems like an ad hoc maneuver to save the solipsistic theory, lacking independent motivation, and therefore dubious on scientific grounds.