All sciences and philosophies are based on some foundational assumptions which are not themselves ever proven to be right, they are just the starting point for the thought process. For Newton it was "space and time are absolute".

Whenever people argue from different foundational assumption, they will always be arguing past each other because they begin with different foundational assumptions. As Kuhn explained, this is a paradigm debate, and paradigm debates cannot be resolved by the normal rules of logic, because they are pre-logical assumptions that seem so self-evident that people can't imagine how anyone could possibly believe otherwise.

The externalist begins with the naive realist assumption: I see the world around me. The world I see is the world itself. Anyone who would contest what is plainly obvious to anyone must simply be mad!

The internalist notices retinal after-images after seeing a camera flash. He sees visual illusions. At night he dreams up whole worlds of fantasy. At some point his mind "clicks" and he realizes that everything is an internal theater. Then another "click" : Beyond the farthest things we perceive is the inner surface of our true physical skull, beyond which is the real world, of which all this here is a miniature internal replica.

The two views cannot both be right, one is definitely more factually right than the other, it makes specific predictions (there are pictures in the brain) while the other invokes magic to explain after-images hanging in space instead of on our retina, and dreams or hallucinations where there is nothing there to be directly perceived.

But in the vein of your discourse, there are circumstances, e.g. driving a car, where it would be inappropriate to think like an internalist and see the world around you as an illusion, so you become a naive realist till you get safely to your destination.

On the other hand in philosophy class it is inappropriate to be a naive realist, because your job is to educate your fellow man of the true epistemological configuration of the world of experience being contained within the skull.

When talking to your grocer about the price of beef, it is totally irrelevant whether you are thinking like an externalist or internalist. So in that sense, the choice is relative to the situation, as you suggest.

Nevertheless, the world of experience *IS* in fact entirely contained within your head, that view is actually right, even if the other is sometimes convenient.

If the blind men were feeling parts of a dismembered elephant, they'd be right, its like a tree, like a snake, like a wall. But if they are all feeling parts of the same intact elephant, they are all wrong, an elephant is like a snake and a tree and a wall, all at the same time, and in a specific configuration.

Naive realism is only right inside the world in your head, where you do indeed experience your sense data directly, unmediated. But it is wrong in the larger context, because that sense data is not the object that you think it is, it is a picture in your brain, a representation, not reality. And there really are pictures in the head.