by Tim Sommers
The philosophy of one century is the common sense of the next. –Henry Ward Beecher
There are several long-running attempts to give AIs common sense. Or, at least, to build a useable database of “common sense” for AIs. MIT’s Media Lab shut down its “Open Mind Common Sense” project in 2016 after 17 years of collecting common sense, but Wordnet has been up and running in Princeton’s cognitive science lab since 1985 and is still going strong. It is now an independent, noncommercial database run by The Global WordNet Association and, purportedly, contains 12+ megabytes of common sense. The always scary sounding Defense Advanced Research Project Agency (DARPA) has its own Machine Common Sense project – and then there’s Delphi which focuses on “ethical” common sense. There’s probably more.
But common sense is a slippery notion. “A stabbing ‘with’ a cheeseburger,” Delphi has said, “is morally preferable to a stabbing ‘over’ a cheeseburger.” Which seems right, but common sensical? I don’t know. Cows say “Moo” is another example of Delphi’s AI common sense. But isn’t that just ordinary knowledge based on wide-spread (if sometimes second-hand) experience? If you stick a pin into a carrot, another nugget goes, it makes a hole in the carrot not the pin. Is that really what we mean by common sense?
G.E. Moore, along with Russel and Wittgenstein one of founders of analytic philosophy, famously proved the existence of an external world – which does seem common sensical – just by waving his hands about. “I can prove now,” he says, “that two human hands exist…[just] by holding up my two hands, and saying, as I make a certain gesture with the right hand, ‘Here is one hand’, and adding, as I make a certain gesture with the left, ‘and here is another’. How absurd it would be to suggest that I did not know it, but only believed it, and that perhaps it was not the case!”
I assume his appeal is ultimately to common sense – and not just his hands.
Which brings me to a paradox that occurred me when I first began studying philosophy that I still can’t quite shake. It’s this.
A philosophical theory can either go against common sense or it can support or justify common sense. Supporting common sense seems pointless. After all, what you are trying to prove is, by definition, already commonly recognized as the sensible view. But if you, instead, challenge or attack common sense what resources or knowledge can philosophy bring to bear powerful enough to overturn or undermine common sense – that which, again, everyone already knows to be the case?
This is no doubt an oversimplification – and a false dilemma. (Aren’t there other things to do in philosophy than attack or defend common sense?) Still, philosophers do, in fact, try do both things: justify or attack common sense. And I wonder still why, and how, either is worthwhile.
The common sense Moore was defending suggests that we live in a spatially extended world, external to our own inner world, with at least four dimensions – and that we are not in a dream or a computer simulation – nor are we a brain in a vat. Philosophers, however, have often argued that either there isn’t an external world (Berkeley, for example) or that we can’t prove that there is an external world (Kant, for example). Some philosophers (Bostrom, for example) have even argued that it is overwhelmingly likely that, in fact, we do live in a simulation right now and not in the external world.
Bostrom’s “Simulation Argument” suggests that it is very likely that in the future there will be many simulations of our world that will be powerful enough to be indistinguishable from reality. And yet there will always only be one base-line reality. But then, the argument goes, we are much more likely to be in one of the many simulations than in the singular, multi-dimensional, “real” world.
One of the most influential philosophers of the twentieth-century, David Lewis, had a trove of new material released this year (he died in 2001), including at least one lecture where he defends common sense.
“I am a conservative philosopher,” he says. “I think philosophy has no business challenging the positive convictions of common sense, and no business challenging established results of the natural sciences and mathematics.
“What should a philosopher do if he discovers a proof that he is the Pope [for example?] He should find the flaw in his proof. If he fails, he should conclude that the proof has a flaw he cannot find. The same goes, say I, if a philosopher discovers a proof that some truths are also false; or that motion or time or value or consciousness or freedom is unreal; or that it is unthinkable that anything exists outside the mind, or outside the text; or that there are no people, and no swizzle-sticks either; or that it is a wide-open scientific question whether anyone has ever believed anything; or that Cantor’s diagonal argument was fallacious; or that we make worlds with words. We should know a reductio ad absurdum when we see it! It is ever so much more likely that a philosophical argument has gone astray than that any of these things is true.”
Fair enough, right? But wait. What is Lewis’ best-known contribution to philosophy? It’s called “modal realism.” The view is that all possible worlds exist; that, in fact, all possible worlds are just as real as our world and that existence is itself indexical. “Indexical” in this context means that when I say something exists, I just mean it is in the same world that I am. However as long as it is possible, on Lewis’ view, even if it doesn’t exist in my world, it does exist in some other actual world. Some modal realists even believe impossible worlds exist. Very few people find these arguments “common sensical” – or conservative.
How can one defend modal realism?
Some philosophers have argued that our everyday ways of speaking sometimes seem to imply the existence of possible worlds. But the big argument that Lewis makes is this. He says modal realism makes modal logic simpler. And he argues that the mere fact that something is theoretically useful or more parsimonious than alternatives can be an ontological argument for it – an argument in favor of its existence.
Brian Leiter’s reaction to Lewis’ conservativism is to reference Gramsci who argued, according to Guido Liguori, “There exist more than one ‘common senses’ distinguishable by area, social stratum and period, continually enriched with scientific notions, and standing in-between folklore and the philosophy of the scholars. It is a ‘disorderly aggregate of philosophical conceptions’ in which ‘whatever one likes’ can be found.”
When teaching Social Psychology, Stacey Holland (aka my wife) used to do the following on the first day of class. She would ask students to volunteer various common sensical insights into human psychology to illustrate common sense can’t be relied on to predict human behavior. For example, birds of a feather, as we all know, flock together. But don’t opposites also attract? Can these both be common sense?
“Common sense is not so common,” Voltaire said. But maybe, that’s wrong. Maybe common sense is much, much too common.
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I have a critique of the simulation argument here.
I once stood in line behind David Lewis waiting for dinner at an American Philosophical Association (APA) Eastern District Meeting. It was a slow-moving line, and I wanted to get Lewis to talk to me, so I tapped him on the shoulder and asked him if there were possible worlds where I was in front of him in that line. He said nothing. He just turned back around. So, I spoke to David Lewis once. But he never spoke to me. True story.