Consensus Gentium vs Ad Populum

The consensus gentium is “the common understanding of all mankind.” An ad populum is an argument of the form “Many believe P to be true; therefore, P is true.” It is generally acknowledged that ad populum arguments are materially fallacious.

Did you notice that when I said “it is generally acknowledged that ad populum arguments are fallacious” I was appealing to the general consensus?

Anyway, as I will never cease to remind anyone and everyone who will listen: material or informal fallacies are fallacious by material or content and not by form.  An argument which has the same form as material fallacy, may or may not be fallacious, depending on the content of the argument. It’s the content that makes it a fallacy, if it is one.  It’s a fallacy of composition to argue “Every brick in the wall weighs 2 kg, so the wall weighs 2 kg” but not to argue “Every brick in the wall has mass, so the wall has mass.” Sometimes, an inference from part to whole is perfectly valid; sometimes it’s not. That is why we have to check things that have the form of material fallacy like “This is true of the each part, so it’s true of the whole” on a case by case basis.  We don’t have to do that with formal fallacies: they are always invalid.  In the case of a material fallacy, we have to ask: under what circumstances is in not fallacious?

In what circumstances might an appeal to popular consensus not be fallacious? Perhaps in exactly the cases that the term consensus gentium refers to.

Philosopher Thomas Kelly argues that an appeal to the consensus gentium may be usefully understood as an argument to the best explanation.  It would go something like this:

  1. Almost everyone believes that P is true.
  2. The best explanation why almost everyone believes that P is true, is that P is true.
  3. So, P is very probably true.

Suppose you were transported to a new country, about which you knew nothing. Suppose that you began asking the people about their country.  Suppose everyone you asked, except one man, said that the ruler of country was King Xerxes, while the one man said that he was the king.  Suppose further than no one obeyed or listened to the man who claimed to be the king, and all told you he was crazy, having the delusion of being the king.  Now ask yourself: what is the best explanation for this state of affairs? It is, of course, logically possible that the one lone man really is the king, and everyone else is (for some reason) either unaware of this or engaged in a conspiracy to fool you or other fantastic things—but surely the best, most plausible, most reasonable, and therefore most likely to be true explanation of everyone but one man saying Xerxes is the king, is that it is true.  And the reason they say the lone dissenter is mad, is that he is mad.  And the reason he is a lone dissenter from the consensus is that he is mad.

Would it be rational to suspend your judgment about who the king is, because (so you reason) the consensus-of-everyone-but-one-man is an ad populum fallacy?

An example I like to use of the consensus gentium is the existence of motion.  It seems (close to) evident that at least some things move.  But this has been disputed by certain philosophers, most notably Parmenides and his disciple Zeno, who attempts to show, by an ingenious series of paradoxes, that any belief in motion results in a self-contradiction.

Zeno’s paradoxes are known to have two properties: (1) almost no one believes that they show what Zeno says they show, that motion does not exist; and (2) they are nearly impossible to refute in a non-question-begging manner. Aristotle says that thinking about them gives one a stomach ache.  Aristotle says this! Aristotle, who never met a thing he didn’t enjoy thinking about! Aristotle does refute them, but he requires a good deal of metaphysics to do it.  If you don’t accept Aristotelian metaphysics, you might have to hold either with Zeno that motion doesn’t exist, or with Hegel (and Marx) that because motion does exist, contradictions exist.

Calculus, by the way, provides no solution to Zeno’s paradoxes, because calculus stipulates that what Zeno argues cannot actually happen, happens; calculus does not require anyone to actually perform an infinite summation—because, as Zeno would be the first to point out, that cannot be done.  Calculus instead treats the limit to which the infinite sum is converging as the same as the result of the infinite sum.  Calculus says of an infinite sum “Let it have been done.” As an operational technique, this works fine. Is the limit equal to the infinite  sum? Well, we don’t actually know: BECAUSE NO ONE CAN DO AN INFINITE SUM TO CHECK.

The nearly universal response to Zeno’s paradoxes is “There is something wrong with his reasoning, even if I don’t know what it is, and can’t say what it is, and I will continue to believe in motion in spite of Zeno’s arguments, whether or not I can adequately answer them.” Descartes is even contemptuously dismissive of Aristotle’s attempt to refute Zeno:

When people say that motion, something perfectly familiar to everyone, is ‘the actuality of a potential being, in so far as it is potential,’ do they not give the impressing of uttering magic words which have a hidden meaning beyond the grasp of the human mind? For who can understand these expressions? Who does not know what motion is?

Descartes is being a dick, and Aristotle’s definition of motion is both perfectly intelligible and, in fact, correct, but my point here is what Descartes is DOING.  He is saying that “everybody knows what motion is,” and even though he doesn’t say this, since it is taken completely for granted, “everybody knows that motion exists.”

Is this response to Zeno an irrational one?

If so, why is it irrational? Is it really true that we ought or must give literally every view or opinion a serious hearing?  A total inability to discriminate here does not seem particularly rational.  But what are we doing, if not appealing, however tacitly or explicitly, to the common basis of shared human experience? Is such an appeal really fallacious?

Another case in point is eliminative materialism.  This is the view that neither minds nor consciousness nor anything like it do presently or have ever existed.  Again, this seems to be something that is refuted directly by all human experience: cogito sum, case closed, end of discussion.

And yet eliminative materialism is the position maintained by some philosophers, such as Paul and Patricia Churchland.  Do the Churchlands really believe what they say they do?  (Or, not “believe” it, since that would be an act of the mind—I don’t know what word they use instead of “believe”).  I am afraid I have difficult believing they are sincere; I believe that they not only have beliefs, but that they know that they do, and don’t really mean what they say.

So certain am I of the falsity of eliminative materialism, I would use it as a reductio ad absurdum.  In fact, I do: I hold that any form of materialism, thought through, entails eliminitive materialism, which is self-evidently false, leading to the conclusion that all forms of materialism are false.

A third example would be the idea that time has three “parts” (or “phases” or whatever the best word is): past, present, and future.

What should we think of someone who denied this? Perhaps someone who maintained there is a present and a future, but no past? That all talk of “the past” is delusional nonsense? Very obviously, we can’t show him the past.  It’s gone. Anything “from the past” we could appeal to, physical thing or memory would be something present in the present.  How could a present thing be direct evidence of the past, unless we already know time has that structure?

It all these cases, it is not clear to me how would we establish the truth of these things except on the basis of belief that our cognitive faculties are basically sound and that, overwhelmingly, human beings form these beliefs: there is motion, we have (or are) minds, and that time divides into past, present, and future.

These things seem to be among the primary data of human experience.  And if an experience truly is a universally human one, does it even make sense to challenge it? Isn’t it part of the data of the world that we seek to explain?

Am I really making an ad populum fallacy when I appeal to the common consent of mankind about such matters? It seems clear to me that I am not.  As my readers know, one of the few things that bothers me more than logical illiteracy, is half-educated logical incompetence, where someone has learned the names of the material fallacies from, and then rampages across social media sperging out about “fallacy” after “fallacy,” NOT UNDERSTANDING that material fallacies aren’t formal, NOT UNDERSTANDING that just calling something a fallacy makes it a fallacy, NOT UNDERSTANDING that something  can have the same form as a material fallacy, and still not BE a fallacy.  I really get to the point where I post an Euler circle diagram, because I don’t know how to communicate the point in a simpler form:


Following Kelly, specifically about an appeal to consensus gentium, I would say,

If everyone, with the exception of a few sophists or philosophers trying to make a bizarre case, believes that P (e.g. we suspect they are lying about not believing that P), the best explanation for the (near) universal belief that P, is that P is true, on the assumption that human cognitive faculties are basically reliable.



Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s