Temperature

Temperature is a feature of a material whose value is what a thermometer reads when placed in contact with that material. It does not depend on the amount of material, only on its condition. A thimble of water at room temperature and a swimming pool of water at room temperature read the same.

Temperature takes its place among the core features of a phase, alongside pressure, volume, mass, energy, entropy, and the rest. Each of these is a feature whose value can be measured. Temperature has a particular role: it is one of the conditions a system can be in. States of a system can differ in their values of suitably chosen physical quantities such as pressure, temperature, and amount of substance. Two samples at the same temperature are, in that respect, in the same condition.

What a temperature reading is not, in this approach, is a story about what is happening underneath. We do not say what temperature "really is" by appealing to some smaller world. We say what temperature is by saying what it does and how we get its value. It is what a thermometer reads. Whether you trust a particular thermometer is a different question, answerable by comparing it to other thermometers and watching how it behaves; this is just measurement, with all the discipline that measurement requires.

Temperature is also one of the features whose differences drive change. A hot object placed against a cold one will not stay that way. The two values move toward each other until they meet at a common reading, and the act of moving is something we can observe and measure as it happens. Temperature, like density, does double duty: it characterizes a material's condition, and where its value differs across a system, change tends to follow.