John Calvin Commentary Genesis 37:36

"And the Midianites sold him into Egypt unto Potiphar, an officer of Pharaoh`s, the captain of the guard." — Genesis 37:36 (ASV)

And the Midianites sold him into Egypt. It was a sad spectacle that Joseph should be driven in this way from one hand to another. For it added no small indignity to his former suffering that he was put up for sale as a slave. The Lord, however, did not cease to care for him.

God even allowed him to be transferred from hand to hand, so that, in the end, it would indeed be clear that he had come, by celestial guidance, to the very dominion that had been promised to him in his dreams.

Potiphar is called a eunuch, not because he actually was one, but because, among Eastern peoples, it was customary to use that name for the satraps and princes of the court. The Hebrews do not agree about the rank Moses ascribes to him; for some explain it as the “chief of the slaughterers,” and the Greek interpreters follow this interpretation.

However, I am more inclined to agree with others who say that he was “the prefect of the soldiers.” This does not mean he commanded the whole army, but rather that he had the royal troops under his command and authority. This role is similar to that of modern captains of the guard, if you also include the office that prefects of the prison exercise. For this may be inferred from Genesis 39:1.

CHAPTER 38.