John Calvin Commentary Genesis 9:4

"But flesh with the life thereof, [which is] the blood thereof, shall ye not eat." — Genesis 9:4 (ASV)

But flesh with the life thereof, which is the blood thereof. Some explain this passage this way: ‘You may not eat a limb cut off from a living animal,’ which is too insignificant. However, since there is no connecting conjunction between the two words, blood and life, I do not doubt that Moses, speaking of the life, added the word blood as an explanation, as if he meant to say that flesh is in some sense devoured with its life when it is eaten saturated with its own blood.

Therefore, life and blood do not stand for different things, but for the same; this is not because blood is in itself the life, but because the vital spirits mainly reside in the blood, it is, as far as we can perceive, a sign that represents life. And this is clearly stated so that people may feel greater horror at eating blood. For if it is a savage and barbaric thing to devour lives, or to swallow down living flesh, people reveal their brutality by eating blood.

Moreover, the purpose of this prohibition is quite clear: God intends to accustom people to gentleness through abstinence from the blood of animals. But if they should become uncontrolled and reckless in eating wild animals, they would eventually not refrain from shedding even human blood.

Yet we must remember that this restriction was part of the old law. Therefore, what Tertullian reports—that in his time it was unlawful among Christians to taste the blood of cattle—smacks of superstition. For the apostles, in commanding the Gentiles to observe this rite for a short time, did not intend to instill a moral doubt into their consciences, but only to prevent the liberty which was otherwise sacred from becoming a cause of offense to the ignorant and the weak.