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Things appear to someone may not appear the same to others.

For instance, people learn the color red through their visions.

Person A recognizes the color of strawberry and tells Person B it is red, in which case Person B agrees with A.

However, the answer from B would not change even if B is color deficient because B knows that the color of strawberry is red and that the color of any object similar to strawberry is red.

Although the definitions of the color red vary from person to person, they will eventually all point to one thing.

Thus, color can be considered a type of illusion.

The perception is independent for individual.

It is impossible for one to fully understand the perception of the others.

The idea of qualia, raw feeling, is also introduced.

For example, an alien who cannot feel pain may have a difficulty understand how pain is felt by humans.

It can either be that they never experience the sense of pain or it is physically impossible for them to feel pain, or they interpreted the pain differently.

In this case, humans and aliens have an explanatory gap.