There are many flavors of this form of skepticism, but consider the brain in a vat thought experiment:
- You are sleeping one night, when the scientist takes out your brain, puts it in a vat, and connects its neurons to a machine that gives/receives electrical inputs such that the brain activity is identical to certain possible real life scenarios if the brain was still in the body.
- The brain in the vat could be having the same experiences I am having right now, so why can I be confident that I am not a brain in a vat?