A homeostat is something that keeps something the same. That works to defend some state.
The simplest example is a thermostat. A homeostat that keeps the temperature the same.
In the high latitudes of East Anglia, in the beastly northern winter, it can be so cold that people need to heat their houses.
If the temperature is too high inside the house, the thermostat turns off the central heating. Heat goes out through the walls.
If the temperature is too low inside the house, the thermostat turns the central heating on. Fuel burns.
The temperature of the house stays constant inside.
Various amounts of heat go out through the walls.
Various amounts of fuel are burnt. But the temperature of the house stays the same.
If your house is too hot, you might reason:
Calories In, Calories Out.
I need to restrict the amount of fuel coming into my house.
I need to remove some of my loft insulation.
It's simple physics.
Amazingly, neither of these approaches will work very well.
You take your loft insulation out. Calories Out goes up remarkably!
And the temperature of the house stays the same.
You take a pair of pliers to your fuel pipe, so that the oil cannot flow into the boiler so fast.
The boiler becomes less effective. It cannot burn as much fuel. It cannot generate as much heat. Calories In goes way down!
And the temperature of the house stays the same.
You have an angry argument with your wife, who claims that your methods are ineffective and probably harmful.
Fuck's sake, woman, you say. It's Physics. The one science we can trust. Take it to the bank. It's a Fucking Law of Fucking Physics.
Your wife quietly adjusts the thermostat so that it turns the boiler on at a more comfortable temperature.
And the temperature of the house drops.
Amazingly, your paper on all this is rejected from Nature on the basis that no physicists are remotely surprised by any of it.
And rejected from the BMJ on the basis that it’s too difficult and what are all these weird squiggles anyway. Is that Greek?