“In statistical mechanics, in other words, we think that there actually is some particular classical state of all the particles, but we don’t know it, all we have is a distribution of probabilities. Happily, such a distribution is all we need to do a great deal of useful physics, since it fixes properties such as the temperature and pressure of the system. But the distribution isn’t a complete description of the system; it’s simply a reflection of what we know (or don’t) about it. To tag this distinction with philosophical buzzwords, in statistical mechanics the probability distribution is an epistemic notion—describing the state of our knowledge—rather than an ontological one—describing some objective feature of reality. Epistemology is the study of knowledge; ontology is the study of what is real.”