The principle of four humors

A common peeve of mine is the tendency for fantasy writing to rely on real science to inform how the fantasy world works rather than on the obsolete scientific theories believed by the classical philosophers. So forget everything that you know about real world biology. This is fantasyland, where real physics take a back seat to fantasy physics!

As recounted in the Descending Stair cosmology, the "[descent of energy through the cosmos] explodes into the lower regions in the form of the Positive Energy Plane, which then splits into the four Elemental Planes. These planes provide the raw ingredients to form the Material Plane" (Classic Play: The Book of the Planes).

In turn, the fleshy bodies of humans and mortal animals are composed of the four humors: blood, choler, melancholy, and phlegm. Each humor corresponds to one of the four classical elements: blood to air, choler to fire, melancholy to earth, phlegm to water. Sickness and even death occurs when these humors are imbalanced. Imbalances may be caused by evil spirits or failure to properly nourish the humors.

Living creatures require nourishment because their inner fire, choler, consumes the other humors. Failure to restore these humors by breathing, eating and drinking results in the choler consuming itself and dying. Choler, by its nature, is quite temperamental. It must be treated no different than the holy fire in a temple. It must be offered the correct sacrifices, for failure to do so results in it scourging the body as punishment (sages call this "malnutrition").

This also neatly explains why so-called "living constructs" would still require a breathable atmosphere and an external source of sustenance: the artificial choler that powers them is little different than the choler of fleshy beings and subject to the same limitations.