I was talking with my roommate over dinner and the conversation turned to mistakes in reasoning. He pointed out to me that the opening line of the Nichomachean Ethics is fallacious. It is: “Every art and every inquiry, and similarly every action and pursuit, is thought to aim at some good; and for this reason the good has rightly been declared to be that at which all things aim.” The fallacy is inferring from \forall x \exists y to \exists y\forall x. Having never read the Nichomachean Ethics (We read the Eudemian Ethics in my Greek Philosophy class for some reason.), I was shocked. Were those the perils of syllogistic reasoning?

I think another good fallacy is in the Critique of Pure Reason. In the paralogisms of pure reason (B422-423), in a footnote on Descartes, Kant seems to make the modal fallacy (is there a Latin name for this?) of inferring from [](p->q) and p to []q.
The quote is, “I cannot say ‘Everything that thinks, exists’; for then the property of thinking would make all beings possessing it into necessary beings.”
Again, I was surprised when I saw that.

What other great fallacies are there…