An old device called a stereopticon held two photographs taken from closely related viewpoints, such that on looking into it the observer saw a three-dimensional view of the photographed scene. This proves that we unconsciously construct, in our brains, a three-dimensional space out of two two-dimensional images, one per retina. Also, if you have someone hold up a finger, it is easy to bring your finger down on to its tip, but if you try this with one eye closed it is difficult -- proving that two eyes are necessary for seeing three-dimensional space. But this means that our three-dimensional visual space is inside our heads, whereas we clearly experience it as out side our heads. So which is it?

I see the laptop on my desk. This seeing entirely depends on some fabulously complicated neural shenanigans in my head. The three-dimensional laptop certainly isn't in my head. But what about my perception of the laptop -- is that in my head? I make a fist. Its existence entirely depends on the operation ofparts of my body. Is the fist inside my body or part of it, as my liver is?

Nothing gets "constructed in your brain". This is a loose (if natural) way of speaking that can get you into trouble. If we open your brain, we will find nothing there beyond neural matter -- no images, no portions of visual space. The activity of your brain does make possible your apprehension of the world around you, but it's certainly not to be identified with what's apprehended. The three-dimensional world that I perceive does not exist in my brain, optic nerves, retinas, etc., even though it's true that I could not perceive it without the latter. In fact, I would go on to say that my perception of the world also doesn't exist in my brain, optic nerves, retinas, etc., though I also agree that those perceptions would not exist without all that neural activity. (For a related discussion, see Question 890.)

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