The skeptic's dependence on common sense

It seems to me that those who argue for external world skepticism seem to rely on the reality of an external world, or at least our common sense notions about it.

The skeptic will argue that we cannot be certain of the existence of an external world. To make this argument, she may point to the possibility of hallucination.

I look out the window and see my dog, Thor. The skeptic will argue that if my seeing Thor were a hallucination, then that experience would be exactly the same as it would be if I were actually seeing Thor. The argument here is that the hallucination and the real experience are identical. If I can't differentiate between the hallucination and the reality, then I can't know whether or not all of my experience is a hallucination. In other words, I can't know it is the external world that I perceive. It may all be a hallucination.

To be honest, I am unsure exactly how to state the claim I want to make here, except to say that in order for the skeptic's argument to work we have to have some prior knowledge of the existence and reality of an external world. The distinction that the skeptic makes between the experience of an external world and a hallucination of an external world depends (trivially?) on our having the ability to make the distinction between the two. But, how could we make such a distinction if there were no distinction to make? It seems to me that it is precisely my prior ability to distinguish between reality and hallucination that allows me to understand the distinction that the skeptic requires.

I have had the experience of seeing something that wasn't there. I have even experienced hallucinations (don't ask). My point is: I know the difference. And, it is in knowing that difference that gives some sense to the skeptic's argument. But, it is also the knowledge of that difference that undercuts the conclusion the skeptic hopes I will accept.

Now, to be fair, I am not completely convinced by the claim I am trying to make here. But, my inclination is to say that if all of our experiences were a hallucination, then we would never be able to make sense of the distinction between the experience of an external world and a hallucination of the same. It is because we have experienced something of both that the distinction makes any sense. The skeptic will surely respond that if all of our experiences were a hallucination we would not know they were a hallucination. And, that is precisely my point. Nonetheless, if I were convinced by this point, then I would have a much higher level of confidence about it than I do. 😊

Whatever the case, I have no doubt there is an external world.