1. I can now give a large number of different proofs [for the
existence of the external world], each of which is a perfectly
rigorous proof; and that at many other times I have been in a
position to give many others. I can prove now, for instance, that
two human hands exist. How? By holding up my two hands, and saying,
as I make a certain gesture with the right hand, Here is one
hand, and adding, as I make a certain gesture with the left, and
here is another. And if, by doing this, I have proved ipso facto
the existence of external things, you will all see that I can also
do it now in numbers of other ways: there is no need to multiply
examples.

2. The Matrix presented a scenario where people were tricked
into believing that the world that they perceived was actually real
when it was in fact fake. If the Matrix was possible, then we would
have no way to tell whether or not we are in the Matrix or not. The
problem is that the Matrix is completely possible, so we are not in
a position to determine whether or not the external world is real
or a computer-created illusion.

3. In what might be called the epistemological tradition,
perception (including such inner perception as consciousness) and
memory are considered experiential sources of knowledge and
justification, whereas reason is viewed as a distinct kind of
source. It might be called intuitive, ratiocinative, or simply
rational.