Claude Bernard, a French physiologist in the 1800s, used the phrase "fixité de milieu interieur" (constancy of the internal environment) to describe the remarkably consistent internal conditions of the human organism—blood pressure, temperature, blood sugar level, etc.

Most of a century later, Walter Cannon at Harvard was intrigued by Bernard's work but disagreed with the rigidity implied by his phrase. Cannon saw the organism not in a "fixed" internal state, but rather in a dynamic and self-regulating equilibrium, and he substituted the term "homeostasis"—literally, "keeping itself the same." (This isn't really so different from "fixité," but you can see what he meant.)

Cannon figured out the mechanisms by which the body maintains an even temperature: if external heat causes the body's internal temperature to rise, the increased temperature of the blood and tissues triggers a series of responses—blood vessels in the skin dilate, sweating begins, and other changes take place, all tending to dissipate heat and return the temperature toward normal.

In a cold environment, opposite changes happen: vessels constrict, the muscle action of shivering generates heat, sweat glands shut down. These changes conserve heat within the body, thus moving the temperature toward normal.

"Normal" implies the range of a variable (such as temperature or blood pressure) within which the organism can live and function.

The essential feature in these control processes is negative feedback. That is, a change in temperature sets off one or more changes that oppose the original change.

Even before Bernard and Cannon, mechanical control systems had been invented using the negative-feedback principle, such as the "fly-ball" governor on James Watt's steam engine in 1769, and a float valve in a tank. In modern industrial and non-organic systems, it is called "cybernetics," or more broadly, "systems theory."

It is a natural step, from the above discussion, to think of homeostasis as a means of control, and to use a simple control mechanism such as a thermostat as an analogy and a model for studying or teaching about homeostasis. Indeed the thermostat model is used in almost every book or discussion on homeostasis—for good reason, but also at the risk of obscuring some grander and more general truths, as I have gradually learned.

A thermostat is designed to regulate the temperature in a room, tank, oven, or other confined place—such as our biology lab. Picture a winter day, a cozy room, a thermostat on the wall, and an oil furnace in the basement. The thermostat is connected by wires to the furnace. Inside the thermostat is a strip of two different metals with different tendencies to expand with warming, fastened together face to face. This bi-metal strip responds to a fall in temperature by flexing, and to a rise in temperature by unflexing, thus closing or opening an electric circuit that turns the furnace on or off. Heat is gradually lost from the room through walls and windows. Oil is available to the furnace from a tank.

The elements of the thermostatic system form a closed-loop negative-feedback system: the bimetal strip; the wires and contacts; the switch which turns the furnace on or off; the furnace with its oil flame; the ducts through which hot air gets into the room; and the temperature gradient that causes heat to move out through the walls (if it's colder outside than inside). The result of such a system is an oscillating, zig-zag action.

It is conventional to think of the thermostat as the "sensor," and the furnace as the "effector."

My own understanding of homeostasis has progressed in a series of steps, and is by no means complete. My learning was facilitated—as so often happens—by trying to teach a brief course on homeostasis. Our laboratory had the built-in thermostat and furnace arrangement, and the wavy red line on a recording thermometer on the table displayed the workings of the system in our room.

One day a truck drove up outside the window. The driver lifted a metal plate in the yard, inspected a gauge beneath it, then pulled a hose from the back of the truck and filled the oil tank which fed our furnace.

This, obviously, was another negative feedback system: a big oil supplier's tank, and a lot of smaller tanks like ours all over town; the oil level in the big tank affected by transfers to the small tanks until the big tank itself needed to be refilled.

Here was the lesson: every system is composed of smaller systems, right down to the smallest particles; and every system is a component of a larger system, right on up to the Whole Shebang.

A few students had difficulty grasping the closed-loop, negative-feedback concept, so I devised a game as a model.

Six chairs stood in a circle, one for each student. On each chair was a numbered instruction card. The instructions on the cards were:

1. When the person on your left holds his card by the red end, turnyour thumbs up. When he holds it by the green end, point your thumbs down.
2. When the person to your left turns his/her thumbs up, turn on your flashlight. When thumbs are down, turn off the light.
3. When the person to your left has the light on, open and close your scissors repeatedly. When the light is off, stop moving the scissors.
4. When the person to your left is opening and closing the scissors, raise your flag. When the scissors are still, lower the flag.
5. When the person to your left is holding up the flag, place the lighted burner under the beaker of water. When the flag is lowered, remove the burner.
6. When the thermometer in the water reads above 100 degrees, hold your card by the green end. When it falls below 100 degrees, hold the card by the red end.

The students sat in the assigned chairs and read the cards, and the "system" surged into motion. It did indeed keep the water temperature around 100 degrees, though a cycle took quite a while—during which time a sudden insight flashed through my mind and I saw some of the flaws in my thinking about homeostasis: the circle of chairs was unnecessary, as were the numbers on the cards.

After class I re-wrote the instruction cards:

1. Look around the room. If you see a flashlight on, turn your thumbs down. If the light is off, point thumbs up.
2. Look around the room. If you see someone with thumbs pointing down, lower your flag. If thumbs are up, raise the flag.
3. Look around the room. If you see someone holding up a flag, turn your flashlight on. If the flag is down, turn your light off.
4. Look around the room. If you see someone with a flag raised, open and close your scissors. When the flag is down, stop moving the scissors.
5. Look around the room. If you see someone with a flashlight turned off, hold your card by the green end. If the light is on, hold it by the red end.
6. Look around the room. If you see someone with a flag raised, tap your stick on the floor. If the flag is lowered, stop tapping and close your eyes.

In this new set of cards, note the major differences:

1. The temperature, or "thermostatic" feature, has been deleted, because by suggesting control it obscured the true nature of the system. The concept of "control" is an arbitrary feature imposed by the human mind on the system, which in itself is neutral and without any "intent."
2. The need to number the cards and match them with chairs in a preconceived pattern no longer exists. To further free up our thinking, drop the numbers and, in random order, name the entities Thumbs, Flag, Card, Scissors, Light, and Stick.
3. There are three categories of entities in the system as it is now structured:
   
   • » Thumbs, Flag, and Light are parts of a closed negative-feedback loop.
   • » Card and Scissors will follow the actions of Light and Flag, respectively, but will have no effect on the loop.
   • » Stick will respond once to Flag and thus be rendered blind and inactive.

It is conventional to think of the thermostat as a "sensor" and the furnace as an "effector." Which here is "sensor" and which is "effector"? Yes, each is both, depending on how you wish to think of it. And indeed the same is true with each part of the thermostat-and-furnace system. The heated air in the furnace "senses" the push of the fan and obediently blows into the room.

The human mind, in order better to understand, needs to divide and categorize information, assign preconceived purposes and uses, and try to make simple order out of an infinite set of possibilities. Sometimes this can obscure broader and more general truths and obstruct understanding.

So let us revise the game once more. Instead of the classroom, let Light, Thumbs, and Flag be set loose in a large crowded space in the midst of a lively cocktail party. Add to their instructions, "If you see [Thumbs, Flag, Light], take two steps toward it and . . ."

We may now expect to find eventually, amid the milling crowd, a group of three people close together, alternately lowering and raising a flag, turning a light on and off, and pointing thumbs up and down. A functioning system has created itself!

Here at last is revealed the real nature and effect of homeostasis. (Not its "purpose." Purpose has no place or meaning here. Purpose, or intent, is a purely human property, though it is often wrongly mixed up with the definition of homeostasis.)

The grand discovery here is that homeostasis not only is the essential principle that makes any system able to survive and continue, but is what allowed the system to come into existence in the first place. Homeostasis is the great creative principle to which I owe my own existence.

Harold Morowitz, in his book, The Emergence of Everything, describes, in 28 rather arbitrary steps, how from the Big Bang all the way to life and the human mind, each stage of organization of energy and matter has produced the materials from which the next stage can organize itself, simply because of its inherent nature and properties. Atoms of hydrogen and of oxygen have somewhat different inherent properties. When they have joined into a molecule of water, the properties of the new molecule are entirely different from those of either of the atoms, and enable further variations in organization and actions. And so the whole world is in a constant state of self-creation. Those creative starts or experiments which happen to be supported by homeostasis endure, for moments or for eons. The rest vanish unnoticed—mass extinctions of the not-yet-creations.

Thus homeostasis is the creationism of evolution, cast in a whole new light.