Imagine for the moment that somebody want to build a fire detector and what they had to build it from was a CO2 detector (C) and a thermometer (T). This would work if there was a fire, temperature and carbon dioxide levels should both rise, so it 'sees' a fire. But you could set it off by bringing in a large pan of boiling water and a chunk of dry ice. It would 'see' a fire again, even though it wasn't there, or by dumping a compost pile there. If we added on a humidity gage (H) we could avoid being tricked by a pot of steaming water and say 'Fire is the state when C and T are high but not H' but it could still be fooled by someone bringing in a hot metal cauldron and throwing some dry ice in there and thinking 'Fire'. The thing is it isn't be fooled at all. It never knows if a fire is going or not.
The instrument is just not reliable to measure 'fire quantity' outside of a tight set of circumstances, which is pretty sufficient if its being used in an oven or fireplace but obviously wouldn't be when used next to a compost bin. Same a human eyeball is a bad device for detecting the presence of 580 nm "yellow" light, because as far as your brain is concerned any spot of a certain visual size and time length emitting either 580nm light or a roughly even mix of 530 nm (green) and 630 nm (red) light is yellow. It happens to have two sensors that are each sensitive to actual yellow photons but are most sensitive to red and green, and a third sensor which isn't sensitive to real yellow at all. Both those sensors go off when exposed to a yellow photon, but one goes off when exposed to red and the other to green. So if a pair of yellow photons come in or a red and a green both those sensors will go of but not the blue one, and that's what your brain receives. It isn't 580 nm that is yellow, that is simple a condition that satisfies 'yellow' if its the only wavelength of photons you're recieving. Two lights near each other, red and green, will eventually appear yellow if you back off far enough. Two lights close like that, flicking on and off will appear as a single light blinking red then green, but as you speed that up it will eventually, to you, appear as yellow. Same as you perceive white if there's a whole bunch of colors at once, 'white' is a fictional color too.
I suppose maybe the best analogy would be chicken, and everything that 'taste like chicken', presumably they do not taste like chicken but happen to taste like something that chicken also tastes like. No chicken flavor, merely a flavor that chicken happens to be a very common variety of. Magenta is like a thing that tastes like chicken, but is not chicken or in fact any individual animal, like if you mixed horse and snake meat and got a chicken flavor.
Hopefully that decreased rather than exacerbated confusion.
- Albert Einstein
King of Cairhien 20-7-2
Chancellor of the Landsraad, Archduke of Is'Mod