Why do we need a contrast to recognize a sensation?

Why do we need a contrast to recognize a sensation?

Why do we need a contrast to recognize a sensation? For example -; Think of hearing the same sound since your birth and think that you are hearing it without any variations. We will fail to recognize that we are perceiving a sensation and we won't be able to recognize the sense organ. Iam only 15,Forgive me if my question is fallacious. Thanks.

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