[adrianN]

So were is the line? Is an Ape using sign language on our side of the line, or on the cat's side? At what age does a child cross the line?

[grishka]

On the cat's side. They don't use it to represent abstract ideas.

I can describe to you something you've never seen and have no idea about, yet you'll be able to imagine it and reason about it from my description alone. Apes or dolphins or crows or dogs or other animals we consider smart can't do that. If they could do it, they'd have their own civilization by now. It's a uniquely human trait among all life forms we're aware of.