Why is "thou canst not make one hair white or black" a good justification not to "swear by thy head"?

Question

Matthew 5:36

Neither shalt thou swear by thy head, because thou canst not make one hair white or black.

What is the connection between "thou canst not make one hair white or black" and "thou shalt not swear by thy head"?

I understand "your head" as a pars pro toto for the person’s body, as an injunction not to swear by your life (or, more broadly, the physical integrity of your body). I don’t understand in what sense the fact that you can’t change the color of one hair is related to that?

Some observations/thoughts:

• The majority of human hair grows on the head. So the "thou canst not make one hair white or black" is an elegant stylistic device referring back to the head you are not to swear by.
• Obviously you can make your hair white or black by dying your hair. So "thou canst not make one hair white or black" is probably not to be understood literally.
• Hair turns increasingly white as people age. Maybe "thou canst not make one hair white or black" is to be understood as "some things are outside of your control"? But I am not sure how old people got some 2000 years ago, so "white hair = old" might not have been such a common connection to make at the time. And in some sense, you can make your hair white by just waiting and aging. So I am not sure if "thou canst not make one hair white or black" is to be interpreted as a symbol for unalterable facts such as aging?
• Maybe something got lost in translation? "Thou canst not make one hair white or black" might be idiomatic in some other language and has just been translated literally?

Answer 1

Jesus is implying a lack of control, that swearing by one's head(or life) which was common in those times is worthless because you do not control your own life or when it ends. The implication is you cannot control the color of a single hair, how can you control your life?

You might also reference the passage from Matthew 10:29-30

Are not two sparrows sold for a penny? And not one of them will fall to the ground apart from your Father. But even the hairs of your head are all numbered.

Again we see reference to hair and life and how both fall under the dominion of God.