Suppose a foot thinks to itself, “I’m not a hand, so I don’t belong to this body.” That doesn’t change anything, it is still a part of the body.
And you know that a body doesn’t have just one part. It has a lot of different parts.
Or suppose an ear thinks to itself, “I’m not an eye, so I don’t belong to this body.” But no matter what it thinks, it is still a part of the body.