If your perception is subject to failure, so to is the evidence, no matter how convincing. So yes, we act upon the assumption that reality exists. We both agree with this.
But that doesn’t mean it is true. And all I’m saying is for this very narrow point of what I care most about, Descartes does have a point. I care more about my mind than my foot. I mean, maybe you can think of a better way to frame the argument because I doubt you even disagree. If you have a gun and you are forced to shoot yourself anywhere on your body, would you choose your foot or your brain?
The better counter to me would be to prove external value. Would I sacrifice myself for someone else? If I believe reality doesn’t exist, the answer should presumably be no. If I believe reality does exist, the answer could be yes. Or alternatively, shooting myself in the foot suggests I believe in a causal relationship within reality towards shooting my brain and losing consciousness, which I shouldn’t necessarily believe.
But even then, it’s not that I disbelieve reality, it’s just that I can’t know for certain what’s real outside my mind, so there’s not really any contradiction between acting as if it is real and being uncertain if it is.
All this is doesn’t matter anyway: the point is less you could be a brain in a vat, but rather if you were a brain in a vat, would you be any less you? I don’t think so.
I have more evidence that the real world exists than I do that you are a thinking mind.
I have more evidence that I am a thinking mind than that I do that the real world exists. There’s no point arguing this point it won’t go anywhere.