[Mattasher]

Good points. I think the hidden terms here are "ratio variables" vs "interval variables". It's not obvious that 200 degrees (Kelvin) is twice as hot as 100 degrees. But almost everyone is going to agree that water at 300K feels hotter than water at 280K (though this may not work across materials, as you note).

What we can do with our temperature scale is build a model that says every time we convert n liters of oil into heat and direct it at x liters of water we get a change of temperature of y degrees. Reliably, and with small margin of error.

We can't do this with GPD, and if we declare that a 10% increase in education spending will lead to a 5% increase in GDP over 15 years, we don't have a testable scientific model, we have a guess based on our interpretation of extremely messy data.