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We often perceive by comparison.

In one setting an object differs from its immediate surroundings in such a way that the brain tell us it has one quality.

If the same object is in a different environment, that is that it differs from its surroundings differently than in the first instance then the brain tells us it has a different quality.

What the example is showing is that human perceptions are NOT the same as analytical equipment.

It's designed to tell us that we shouldn't be quite so ready to be certain about our internal constructs until we have tested them.

Hope my comments help.

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