How long has western civilization known the body contains a significant fraction of water?

Question

Body water is the water that makes up more than half of the average human body.

Before the advent of modern physiology, humorism associated Phlegm with water, and my (relatively limited) understanding of the theory suggests that this was supposed to be "balanced" with the other three. I haven't been able to find what that "balance" is, but at least some significant fraction of the human body was water. Humors having been described in the writings of Galen between 130 and 201 AD somewhere is the earliest reference I could find for the human body containing water.

Looking for an earlier scientific reference, I've skimmed De Rerum Natura for references to water, but didn't see it mentioned in particular reference to human anatomy. It describes water as being one of the four traditional elements, but doesn't say whether there's any in the human body.

Is there an earlier reference for the human body's water content? When did western civilization decide humans were wet?

Answer 1

One must be very careful when reading the ancients to not impose modern interpretation, laden as it is with understanding acquired with the aid of the microscope, on ancient texts. Words like atom and element are bandied about, but their meaning is significantly different than today's, aided as it is by chemical understanding wholly obtained in the 19th century.

For instance, in The Nature of Things Lucretius uses the word atom both in places where a modern understanding would require molecule as well as in places where the modern word atom is accurate. Likewise his use of the word element is frequently used in places to denote what we would now call a pure compound. Our modern access to observation through optical and electronic microscopes gives us a much richer base of observations upon which to reason.

Therefore as blood would be regarded by Lucretius as the wettest humour of the body, it would be inconceivable to him to infer a wetness for the body in excess of that of blood. And without the aid of a microscope the highest reasonable wetness that could be inferred for blood would be the 55% plasma content observable by allowing blood to settle at cool temperatures. (A centrifuge speeds this process but is not essential.)

Note that much of the body's water content, more than a third, is wholly contained within the cells of the body and is thus not observable with the naked eye. On page 117 of Watson's translation of Lucretius mist and water are noted as being different substances, though noting that they are one and the same is actually a naked-eye observation that, with care, could be made. Therefore it is inconceivable to me that any ancient could have logically inferred a water content for the body in excess of about 55%.