What is the basic internalist position? John Heil writes:
"Descartes is imagining that your inner life could remain wholly unaffected while the world you changed dramatically…"
This position seems quite counterintuitive. One can immediately see how the "evil demon" and the "brain-in-a-vat" hypotheses immediately follow from this position. Quite simply, such hypotheses wouldn't be possible if the Cartesian position weren't true. It's almost as if one’s mind is a closed cell in which the only access to "external reality" is through a TV screen which nevertheless doesn't allow any direct causal content with the outside world. If that were the case, no wonder the world could radically change without the mind itself also changing. After all, if you watch a scene of war on your TV screen, you aren't really in that scene. Thus there is no need to run and there is no chance of you being shot dead – not even by a stray bullet. As Heil puts it:
"Both the demon possibility and the brain-in-a-vat fable assume that the contents of your thoughts about the world are a wholly internal affair. Your thoughts can have external causes, but their contents – what they are thoughts of – owe their character wholly on your internal constitution. Your thoughts about trees that are indistinguishable from thoughts about trees occurring to a brain in a vat or an agent under the spell of an evil demon."
One can ask here that if the contents of our thoughts "are a wholly internal affair", then how can they be "about the world" at all? The world, on this picture, is seen as external, after all.
Even on the internalist picture we can accept that our "thoughts can have external causes"; though once they are caused, all that matters about such thoughts is their "internal constitution". That is, once they are caused we need no longer know anything about the world or its nature. Even thought these thoughts are indeed about the world, what the world is like now (or its causal interactions with the mind) simply no longer matter when it comes to knowing the contents of our own thoughts. The mind is disengaged from the world and no longer requires it when it comes to mental content and our knowledge of mental content. If the world no longer matters, then of course it's true that a brain-in-a-vat’s thoughts about a tree are indistinguishable from a brain-outside-of-a-vat’s thoughts about the same tree.
From the Cartesian thesis we directly get the brain-in-a-vat possibility. From the latter possibility we can say that of course there will now be a problem: "what reason could we have for believing that our thoughts about the world “matched” the world?" Because the world doesn't matter any more, and because our causal interactions with the world no longer matter (that is, because what the world does to the mind no longer matters), then of course we have a problem of matching up our thoughts with the world. If the world is no longer directly responsible for the contents of our thoughts now, then ‘"or all we know the world could be very different from what we take it to be". Not only that: there may not be an external world at all. Though if the world still matters, and if our causal interactions with the world still determine mental content, then it makes little sense to talk about the "external world" at all and thus these sceptical scenarios would no longer even occur.
What if we couldn’t have a thought about a particular tree if that tree were not causally interacting with us while we were having thoughts about it? If we require such causal interactions with the tree in order to have the thoughts that we're now having about it, then the sceptical hypothesis couldn't even arise in the first place. In order to have a thought about this tree, this tree must exist and must be causally interacting with us.
However, does it also follow that we are correctly matching or representing the tree’s shape? We may need causal interactions with the tree in order to have thoughts about it; though does it follow that what we think or represent correctly matches the shape or reality of the tree? Such causal interactions may bring these thoughts about; though it doesn't follow that our thoughts correctly represent the tree as it truly is. Causation alone doesn't seem to guarantee correct thoughts about the tree. It does, nonetheless, guarantee that we have thoughts about the tree of some kind.