The epistemological idea that only your mind is sure to exist.
Though technically true, I think by bayesian inference, because 1. there are other humans like my species, 2. humans have brains, and 3. brains imply having a mind, the probability that MY mind is the only mind is practically non-existent. Rather, it would straight up contradict so much neuroscience in the first place, so pragmatically it's probably better to assume anything that is pretty much guaranteed; otherwise, the fundamental nature of reality would collapse immediately. (As you don't even know whether any stimulus is real, including perception of your own mind!)