Here (as well as in 1 Corinthians 12:16 and 1 Corinthians 12:21) Paul uses prosopopoeia, an ancient rhetorical device in which an absent person or object is made to speak. He imagines that a human foot might think itself inferior to its more skillful counterpart, the hand. Yet such thinking does not make the foot less important or less part of the body. Likewise, every church member, regardless of what that church member himself might think, is a necessary part of the church.
15 If the foot should say, “Because I am not a hand, I do not belong to the body,” that would not make it any less a part of the body.