Obviously the world is not quite as simple as this, although, at its most basic level, it comes pretty close. So let's now add the next level of complexity to the diagram. What emerges is still very straightforward and obvious. There are no tricks or metaphysical slights-of-hand.
P = Person H = Head B = Brain e = eye L = Light M = Monitor C = Candle F = Flame
P does not actually see the objects M, C, and F. What P sees is the Light L emanating from or reflecting off these objects. L enters P’s eyes (e), moves down the optic nerve to the visual centers of P’s Brain B and thus P sees M, C, and F.
B is the receptacle for all sensory inputs. B sees the objects. The Monitor M and images C and F are outside, the Brain B is inside the Head H. Light L is the medium of exchange. This is a common sense view of what any sighted being does. No surprises or tricks.
P looks at M. M is an external object. How does P know this? Because P accepts that he is not inside a dream. P regards himself as a separate and discrete entity that exist independently and is able to apprehend external objects and distinguish them from internal thoughts. This may or may not be true, but it is not an unreasonable assumption.
At any rate we share a common sense view of what is outside and inside. When we sense the presence of the object, that is inside. When we understand that the object before us is a computer screen, that knowledge is also inside us. If P looks away from M he is reasonably confident that it does not cease to exist. If P leaves and returns tomorrow, chances are good that M will still be there and that it will look essentially the same as the last time P looked at it. This is because M exists separately and externally. Its objective characteristics do not depend on P’s mental state (this is not completely true but it is close enough to the truth for this argument).
Both you and P are looking at Monitor M. If you and P tap on M you have a second sensory confirmation of M’s presence. You may also note that the objective you and P exist in a spatial relationship to M. But where are you and P? Well the Brain you is inside H. Where is H? On the shoulders of you and P. Tap the side of your H just to be sure. Both you and P know where to find H in order to tap on it. For P and the vast majority of you there is indeed an H to tap on. To be sure you are in front of M tap on it to confirm its presence.
Thus you and P establish the existence and presence of M and H and the spatial relationship between them. As you and P can clearly determine, M is external to H. Again grab M and shake it a little. That is M alright and it really is there, external, and outside. Just to be really sure, grab your H. Same conclusion.
We are sorry if this is belaboring the obvious. But in order to proceed we must establish concretely and with some certainty our almost universally shared sense of what is meant by the external world. There is no trick to it and it is fairly obvious. Everyone from Thales to our cat have pretty much looked at the world and reality in this way.
We can haggle over the meaning of terms like Monitor, Head, front, spatial relationship and all the rest. But as was pointed out in the beginning, this is not to be an exercise in words, meanings, and ultimate knowledge. All that we are trying to establish is that when we talk about external reality we are referring to the Ms of this world. We are referring to this M event as literally rather than metaphorically or figuratively external. Whatever the ultimate nature of M's reality, it is experienced in its externality.
