John Calvin Commentary 1 Corinthians 12:21

"And the eye cannot say to the hand, I have no need of thee: or again the head to the feet, I have no need of you." — 1 Corinthians 12:21 (ASV)

And the eye cannot say to the hand. Until now, he has been showing what the role of the less honorable members is—to fulfill their duty to the body and not envy the more distinguished members.

Now, on the other hand, he instructs the more honorable members not to despise the inferior members, whom they cannot do without.

The eye excels the hand, yet it cannot despise it or insult it as if it were useless. He draws an argument from utility to show that it should be this way: “Those members that are less honored are the more necessary; therefore, for the safety of the body, they must not be despised.”

He uses the term weaker here to mean despised, just as in another passage, when he says that he glories in his infirmities (2 Corinthians 12:9), he expresses, through this term, those things that made him contemptible and lowly.