This fallacy goes by many other names as well; The Appeal to the Masses, Argument from Consensus, and the Bandwagon Fallacy, to name a few. This fallacy is easy to spot and goes like this:
“Most people believe X, therefore X.”
Why is this a fallacy? Well it’s almost too obvious to spell out, but this fallacy does come up a lot with non-philosophers. Trainee philosophers still occasionally fall into some of the less obvious fallacies, but this one is hard to use by accident once you know about it. Anyway, what is wrong with this line of reasoning? Well, it assumes that the masses are always right about everything. To quote Brian Dunning, “The majority might sometimes be right, but they’re hardly reliable.”
The appeal to the masses can be considered a subset of the Appeal to Authority since the user considers the masses to be an authority. This is a fallacy for the same reason as the appeal to authority too. Namely, facts about the way the universe is are independent of our beliefs about them, whether it is one authority or most people. This point can be illustrated by reminding you that some really smart people believed things that aren’t true (like Aristotle’s beliefs about the nature of matter) and the majority of human beings have been wrong on issues before (like the ethics of slavery of the shape of the earth). It also falls into the same trap of saying that if the masses changed their mind about the shape of the earth, then the earth would change shape.
Fallacies and Truth
If I haven’t pointed this out before, I need to explain that just because someone uses a fallacy, it doesn’t mean that they are necessarily wrong. Don’t consider this to be a Fully General Argument to defend yourself whenever anyone points out that you have used a fallacy however. If someone points out that you have used a fallacy, the correct response is not, “Well, that doesn’t mean I’m wrong.” It’s true, but that doesn’t support your fallacious argument.
The correct reply to someone pointing out that you’ve employed a fallacy is to make an honest effort to see if they are right. Depending on the fallacy you’re being accused of, this could require some humility and re-assessment of your own argument. The most important thing to do when you’ve used a fallacy is to explain your claim with a different argument. For example, if someone says you’re using the Ad Hominem fallacy you need to explain why that person is wrong by attacking their argument and not the arguer. If someone says you’re using the Appeal to Authority, you need to explain why the authority is right. This might be hard for some higher order scientific facts. For example, I can’t explain why I believe that water consists of H20 without simply saying, “That’s what my textbooks say.”
However, the reason that this is different than blind faith is that in principal I could go out and examine the evidence on my own and, presumably, come to the same conclusion. But on easier questions, like “Is the earth a sphere?” the reason to say “yes” is not because the masses believe it is, but because the earth actually is a sphere and you can go out and verify this if you don’t believe it.
If I say that the sky is blue because my friend told me so I have committed a fallacy, but I’m right by accident. Being right by accident is not the same thing as being rational because my belief about the world doesn’t come from actually observing the world itself and using logic to come to conclusions. If I read in a book that some guy from history is supposed to have existed, but there is no other evidence for it, then holding up the book as evidence is exactly like me saying, “But my friend says the sky is green!” It doesn’t matter what authorities or the masses say, the truth about the universe can only be reliably found by an honest and careful examination of it, and that is what the truth ultimately comes down to.