One of the first distinctions encountered in science education is that substances can assume different states of matter: for example, water can be found as a liquid, as a solid (ice), or as a gas (water vapor).
This concept gradually becomes more and more sophisticated: The state reflects the macroscopic variables of the problem (e.g. pressure, temperature), different substances have different phase diagrams, different phases can coexist under certain conditions, etc. It's thus not surprising that the phase concept is central to modern science and in particular our attempts to discover/create/understand novel forms of matter. (Most discussions of high-temperature superconductivity, for example, revolve around trying to obtain a theoretical understanding of the observed phase diagrams of various materials.)
For the purpose of this question, what I'm specifically interested in is the early history of the phase concept, and what references are available on the subject. For example, roughly when did human beings recognize that liquid water, solid ice, and water vapor could all be understood as different states of the same substance?