Abstract
What reason or justification is there for thinking that there really
is an external (physical) world? What reason or justification is there
for thinking that there is not, instead, an immaterial demon who
deceptively makes it appear to us as if there were real physical
objects? What reason is there for thinking that we are not always
dreaming, that we ever perceive or remember things correctly,
that the humanoid bodies there appear to be around us are the
bodies of conscious beings, or that any sort of God at all exists?
Questions like these have been raised by philosophers of all kinds
over the centuries.