Leviticus 17:14 seems to categorically prohibit the eating of flesh with the blood or the drinking of the blood of any creature:
"For the life of all flesh is in the blood. Therefore I said to the children of Israel: you shall not eat the blood of any flesh at all, because the life of the flesh is in the blood, and whosoever eateth it, shall be cut off."
(anima enim omnis carnis in sanguine est unde dixi filiis Israhel sanguinem universae carnis non comedetis quia anima carnis in sanguine est et quicumque comederit illum interibit)
If this commandment applies, then, so runs the argument, the Eucharist cannot truly be the Body and Blood of our Lord, for to consume it would be against what God commanded in Leviticus, and thus God is made to contradict Himself. What do we say to this?
Well, there are two explanations, one satisfactory, one (in my opinion) more unsatisfactory. We shall look at the less satisfactory of the two defenses to this charge first.
This defense I originally heard from Dr. Scott Hahn
(I do not recall where exactly I heard him give this explanation, but I believe it was on a radio call-in show). According to Dr. Hahn, the response to the charge of cannibalism is "Guilty as charged!" If I am remembering it correctly, Hahn admits that eating flesh and blood is cannibalism, and does not deny that we eat the Body and Blood of our Lord, and therefore asserts that the Eucharist truly is cannibalism. What else could it be if the Eucharist is truly the Body and Blood of our Lord, and cannibalism means eating human flesh and drinking human blood, which the Eucharist is: consuming the flesh and blood of a Divine Human. So, according to this argument, it is a type of cannibalism.
What about the fact that Leviticus specifically says that whoever does this shall be "cut off" from God's covenant? If it is cannibalism, then are we not "cut off"?
Dr. Hahn says that by partaking of the flesh and blood of Christ we do, in fact, want to "cut" ourselves off from the Old Covenant. In accepting the flesh and blood of the God-Man, we implicitly renounce adherence to the Old Covenant, which forbid cannibalism. Christ took upon Himself the curse of the Old Covenant, and we too must accept that "cutting off" from the Old Covenant to be truly accepted into the New. Therefore, we take upon ourselves the reprobation of the Old Covenant which was reserved to those who ate and drank blood by doing the very thing which God prohibited under the Old Law. This we accomplish when we partake of our Lord's sacrifice by receiving His true Body and Blood in the Eucharist. This act definitively "cuts us off" from the Old Covenant and incorporates us into the New.
Well, I guess it makes sense a little bit, but it is novel way of understanding the question. To me this argument is not entirely satisfactory. I prefer the older, more precise Scholastic explanation. This brings us to our second defense, which I think is more sound and begins with the definition of what cannibalism entails, exactly.
According to our second defense, cannibalism is defined as eating the flesh of one's own species under the form of flesh. What do I mean "under the form of flesh?" Well, if I eat a man, say, part of his leg or arm, then I am committing cannibalism: I am eating human flesh under the form of flesh. This is what we usually think of when we speak of cannibalism.
But, let's posit another scenario: Say that a shark eats a man. The unfortunate fellow is digested and becomes part of the shark. Then, a week later, some fishermen catch the shark, kill it and turn it into shark steaks. Then the fishermen eat the shark. Are they committing cannibalism? After all, the man is in the shark and they ate the shark, implicitly eating the man who was eaten by the shark. Is this cannibalism?
No, this is not. But aren't we consuming human flesh when we eat the shark which has eaten the man? Yes, but not under the form of flesh. The flesh of the man has been broken down and is not longer under that form anymore. Therefore, though in a way it can be said that we are eating human flesh when we eat the shark, we are certainly not committing cannibalism. We are eating human flesh in a way, but it is no longer under the form of flesh.
Now, in the Eucharist, we eat the flesh and blood of Jesus Christ under the forms of bread and wine. No flesh is consumed in the form of flesh (i.e., true flesh and blood is eaten, but not in a carnal sense). Therefore, flesh and blood are truly consumed, but there is no cannibalism. I think this is a better and simpler explanation than Dr. Scott Hahn's. Hahn's explanation does not deny the charge of cannibalism and makes up a far-flung and complicated theological reason why it is okay, while the second simply reduces the matter to a confusion over the meaning of the term "cannibalism."
It could be objected that the argument of flesh under the form of flesh only applies when a third party is present to negate the act of cannibalism: the shark eats the human flesh, and then someone eats the shark. However in the Eucharist, bread and wine become flesh and blood, those participating eat it directly (without the intermediary of a third party, a "shark" who removes us one degree from cannibalism).
However, this is not a valid objection. In the Eucharist, there is a "third party", so to speak: the accidental properties of the bread and wine, which remain and render so that the flesh is not consumed under the form of flesh, but under the sacramental signs of bread and wine. There is the Body and Blood of Christ, and us who receive it, but God allows the accidental properties of bread and wine to remain. The presence of these accidental properties are what allows us to say that the Eucharist is the Body and Blood of our Lord under the forms of bread and wine and not under the form of flesh. So, we do not deny that flesh is immediately and directly consumed, but we do deny that it is flesh under the form of flesh.
Thus, the argument that the Eucharist is cannibalism does not hold weight.