The argument cannot be about whether it is possible to think in a natural language. As Hauser points out , we have direct phenomenological evidence of that ability. Furthermore it would be almost incoherent to suppose that we are unable to entertain mental texts expressing thought content given that we can produce physical ones that do that. The argument probably cannot be about whether it is impossible to think without using a natural language either. Convincing thought exercises show that at least some thought is imagistic rather than verbal. (One requires you to imagine a square, and then imagine two lines being drawn diagonally across it from corner to corner, and then imagine horizontal lines across it about a third of the way down from the top and about a third of the way up from the bottom, and then count the partitions of the square.) Note also the shape recognition experiments that suggest that subjects are rotating mental images (see Shepard and Cooper 1982).