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A Moment of Science

The Moving Thumb

Philosophers spend a lot of time pointing out that we don’t really see the world, we see what our brains make of the world. Too much of this kind of thing can make you change your major to Business Administration. But science has shown that those philosophy types actually have a point. Here’s a simple demonstration you can do to show that your brain doesn’t just take in information, but always interprets that information.

Hold your arm out and stick your thumb up in the air. While looking at your thumb, slowly move your hand back and forth. Next, hold your hand still and slowly move your eyes back and forth. Why did your thumb seem to be moving in the first instance and not in the second? The same image was sent to your retina: your thumb, moving back and forth. But in one instance you felt your hand was moving and in another you felt your hand was still.

This shows that our brains aren’t just open windows through which the world pours. They are always interpreting the information they receive. In this case, your brain knew your eyes were moving, and it interpreted the change in visual images as coming from inside your head. When it isn’t told your eyes are moving, your brain reads the same image as motion taking place outside your head. This is the same reason lying down on the bed doesn’t make you immediately panic when you see the room turn on its side. In fact, your brain is always seeing the world move and deciding whether that motion is “in here” or “out there.”

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