Not having a word for 4 and doubting that 2+2=4 aren't the same thing. The former means that you cannot understand what the proposition means, while the latter

Not having a word for 4 and doubting that 2+2=4 aren't the same thing. The former means that you cannot understand what the proposition means, while the latter means that you do understand what it means, but aren't convinced that it's true.