I am in no way doubting that humans know that -273.15 centigrade is 0 Kelvin. I'm just curious.
Suppose you know only two things:
- A new scale for temperature known as the centigrade scale, calibrated against the freezing and boiling points of water using nifty glass thermometers (or any other standard thermometer for that matter).
- Thermodynamics as it was in the mid 1800's - i.e. knowledge that temperature should be a strictly positive quantity.
How do you go from knowing those two things to saying "Aha! I have now inferred that absolute zero-temperature in the modern thermodynamic sense is, on the centigrade scale, at -273.15!"