Well‑conditioned animals have a moderate amount of pure blood, neither overly abundant like recently… — Aristotle

Well‑conditioned animals have a moderate amount of pure blood, neither overly abundant like recently drinking animals nor scanty as in excessively fat animals, because fat is bloodless and increases inversely with blood supply.

By Aristotle, from History of Animals

Key Arguments

He defines the healthy mean: animals "in good condition" have blood "neither too abundant-as creatures just after drinking have the liquid inside them in abundance-nor again very scanty, as is the case with animals when exceedingly fat."
He explains that very fat animals nevertheless have good‑quality blood in small quantity: "For animals in this condition have pure blood, but very little of it, and the fatter an animal gets the less becomes its supply of blood;"
He grounds this inverse relation in an essential property of fat: "for whatsoever is fat is destitute of blood."

Source Quotes

Animals that are internally and externally viviparous are more abundantly supplied with blood than the sanguineous ovipara. Animals that are in good condition, either from natural causes or from their health having been attended to, have the blood neither too abundant-as creatures just after drinking have the liquid inside them in abundance-nor again very scanty, as is the case with animals when exceedingly fat. For animals in this condition have pure blood, but very little of it, and the fatter an animal gets the less becomes its supply of blood; for whatsoever is fat is destitute of blood.

Animals that are in good condition, either from natural causes or from their health having been attended to, have the blood neither too abundant-as creatures just after drinking have the liquid inside them in abundance-nor again very scanty, as is the case with animals when exceedingly fat. For animals in this condition have pure blood, but very little of it, and the fatter an animal gets the less becomes its supply of blood; for whatsoever is fat is destitute of blood. A fat substance is incorruptible, but blood and all things containing it corrupt rapidly, and this property characterizes especially all parts connected with the bones.

Key Concepts

Animals that are in good condition, either from natural causes or from their health having been attended to, have the blood neither too abundant-as creatures just after drinking have the liquid inside them in abundance-nor again very scanty, as is the case with animals when exceedingly fat.
For animals in this condition have pure blood, but very little of it, and the fatter an animal gets the less becomes its supply of blood;
for whatsoever is fat is destitute of blood.

Context

Book III, chapter 19; Aristotle links blood quantity and purity to overall condition and to fatness, connecting this to his earlier discussion of fat.