What did ancient Greek philosophers believe blood was?

As ancient Greek philosophers began to investigate the mysteries of the human body, they were faced with difficult questions about what blood is, how it comes to be, and how it relates to the structures of our body.

For guidance, they looked back to Homer. There is a scene in the fifth book of the Iliad in which one of the Greek heroes, Diomedes, gets his chance to shine. Diomedes strikes at Aphrodite and manages to gouge her with his spear. Homer says that she doesn’t bleed blood; instead, she bleeds “the ichor that courses through their veins, the blessed gods – they eat no bread, they drink no shining wine, and so the gods are bloodless, so we call them deathless” (V.381-384).

Homer’s point is that the gods don’t have blood. They have ichor. Why? Because they don’t eat food. They eat ambrosia, and so their divine bodies produce this other substance, ichor, instead.

Later Greeks, such as Plato and Aristotle, looked back on this as the beginning of an important tradition. Blood comes from the food we eat.

Aristotle (384 - 322 BC) produced evidence in support of this belief.

For instance, in the Parts of Animals, he says that during periods of our lives when we’re eating more, we have more blood, and when we are eating less, we have less blood (II.3 650a31).

This is an attempt to justify a tradition that everyone pretty much already believed in.

When we start digging into the details of how blood is made, things start getting more controversial.

Aristotle maintained that blood is food that had been concocted, or heated, in the heart. It started in the mouth, had been transformed during the digestive tract, and at some point around the jejunum, some of it is transferred into the veins and from there it goes the heart. The rest becomes excrement.

Once it is in the heart, it can be transformed into blood and delivered into the tissues of the body.

Aristotle’s teacher, Plato (428 - 348 BC), had a different view of what is going on.

Plato, in the Timaeus, maintained that blood is food transformed by fire, which at first sounds like Aristotle’s view. However, Plato thinks that the transformation happens in the belly, and the heart isn’t involved. Moreover, Plato thinks that fire isn’t concocting food. Instead, it is cutting it up. As we’ve talked about before, Plato thinks that fire is composed of sharp tetrahedrons, or pyramids.

The action of fire — which you and I call burning or melting — Plato thinks is, actually, cutting up. Fire destroys things by cutting them up into little pieces.

Food sits in our belly and waits for fire to chop it up. It becomes blood that way. It then is transported from the belly into the veins.

In Plato’s view, fire is doing more than merely cutting up blood. It is also dyeing it. Blood should be multicolored, since the foods we eat are multicolored. But blood is red. Why? Plato thinks it’s because the redness of fire has dyed and change its colors.

So, what good is blood doing for us? Well, at some level, every philosopher agrees: blood is that which replenishes our tissues. But things get more difficult when you dig deeper.

Aristotle thought that blood is potentially the tissues of our body. As he says, water is potentially the body of a plant. Bricks are potentially a house. Similarly, blood is potentially our tissues. Our physiology coordinates the transformation of food into blood and then, later, blood into the tissues of our body.

That’s what keeps us alive.

Plato thinks that our tissues are made of the four elements: earth, air, fire, and water. Over time, our tissues become depleted due to environmental influences, like the buffeting of wind. These influences are threats not as obvious as lions, but they are worse because they are inescapable.

As our tissues get worn down by the environment, we need to replenish them somehow.

Thankfully, blood is there for us. Our food is made of the four elements, too, and so when fire in us chops it up, it breaks the food down into the elemental constituents. That’s exactly what our tissues need. So, the veins deliver needed resources to tissues that are falling apart.

Sadly, environmental influences will always win, if nothing else kills us.

Let’s think about it like this. Our tissues are being depleted. Food, transformed into blood, is replenishing those tissues. The fire in us, composed of tetrahedrons, is responsible for those transformations. If the fire isn’t sufficiently efficient at turning food into blood, then we won’t be able to come back from the losses that the environment is imposing on us.

And that’s exactly what happens: as we get older, the fire in us becomes less efficient. The tetrahedrons become dull after a lifetime of chopping food. Maybe they never get perfectly dull, but at least dull enough that our body breaks down.

That’s what Plato thinks death by natural causes is. The environment wears us down, literally.

All of these considerations speak to the importance of blood in the accounts of the most influential Greek philosophers.