Is there an external reality? Of course there's an external reality. The world exists. It's just that we don't see it. At least, we don't see it as it is. In fact, we can never it as it is! In fact it's even useful to not see it as it is. And the reason is because it goes back to really Berkeley, who tells us we have no direct access to that physical world other than through our senses. And because our senses conflate multiple aspects of that world we can never know whether our perceptions are in any way accurate. And so this has always been a very deep question. It's not so much "Do we see the world in the way it really is?" but "Do we actually even see it accurately?" And the answer is no, we don't. So if we remember that the information that's coming on to your eye or onto your skin or into your ears is inherently meaningless (because it could mean anything) then it means that we need another kind of data in order to be able to generate behaviors that are useful. And that data is necessarily historical, which means that the functional structure of your brain is really a physical manifestation of your past interactions with the world, and it's a physical, active interaction. It's not passive, receiving data like a Facebook broadcast; it's an active engagement with that world.