Appeal to popularity
An appeal to popularity is very similar to an appeal to emotion in that it targets emotions; The difference being that it does not focus on the listener's emotions, and instead focuses on what the majority of people think or the popular position to take with regards to the claim. The arguer then uses this to try and persuade the listener to change their mind and/or conform.
A second idea comes from the fact that popularity is often equated with quality. Popularity ratings are given for everything from electronics to restaurants to hotels. The idea is that people have tested out a product or service and we can look to them to give an accurate description of what it is like. If, for example, one hotel is more popular than another, we often assume we would have a better stay at the higher rated/more popular hotel. The arguer of appeal to popularity tries to push the same assumptions in regards to religious belief, where the most popular belief must be the best.
- Classic example: Somebody tries to convince their friend to try smoking by asserting that all the cool kids do it.
- Advertisements that make the claim that: Everybody's doing/using/eating [insert product name here].
- "2.1 Billion Christians can't be wrong."
- Multiple people who independently come to the same conclusion indicates that it's "mind-independent", and thus objectively true.
When it comes to the example that someone says "2.1 billion Christians cannot be wrong" it is easy to point out that more people believe Christianity is wrong than believe it is correct. Citing that two-thirds of the world's population is not Christian would, by their logic, suggest that Christianity is incorrect. In other words, "4.7 billion non-Christians can't be wrong."
The "mind-independent" example above may seem logical on its' surface. If a phenomenon, like Gravity or Evolution, objectively manifested, it's just to be expected that multiple people can discover it.
The problem is in the reversal of this line of thinking; if multiple people "discover" it, it's probably objectively real and true. It's also true that (with accidental exceptions) all dogs have four legs, and multiple people can observe reality and come to this conclusion. That doesn't mean, however, if multiple people count the legs on an object and conclude therefore it's a dog, that they're correct. Other things in reality have four legs too.
Numerous reasons can cause people to arrive at the wrong conclusion, such as:
- Common psychological mechanisms; availability bias, confirmation bias, pareidolia, etc.
- Physicals resulting in a common illusion, such as mirages
- Cultural conditioning
The fact that multiple people come to the same conclusion establishes a correlation, but not the causation - the underlying mechanism which lead people to the same conclusion. It's also an exercise in forming an Argument from Credulity - that what's believed to be true, means it's true.