The Most Common Fallacy? | Affirming the Consequent

If it’s raining, the streets are wet.

The streets are wet.

Therefore, it’s raining.

Seems reasonable, right?

Well… not so fast.

In this episode of Ontology Explained, we take a look at affirming the consequent, one of the most common logical fallacies. I’ll explain exactly what the fallacy is, introduce some useful terminology from formal logic, and show why the argument is invalid.

We’ll look at some classic examples—including one involving the Turing Test and AI—and then tackle a more interesting question: when does an argument that looks like affirming the consequent actually give us good reason to believe its conclusion?

That leads us to explanatory reasoning and inference to the best explanation. The key is understanding the difference between a deductively valid argument and an argument that gives us good evidence.

So: if the streets are wet, did it rain? Maybe. But you can't prove it just from that.