The four terrestrial elements
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Commentary
The four terrestrial elements
The diagram above, although modern in execution, is ancient in origin. It provides a graphic depiction of the way in which the four terrestrial elements of Aristotelian physics -- earth, water, air and fire -- are generated out of two pairs of opposite qualities: hot and cold, wet and dry.
The locus classicus in which this scheme is explained is in Aristotle's work On Generation and Corruption (bk. II, pts 2-3), which explains that:
Fire is hot and dry;
Air (ἀτμὶς, like vapour) is hot and wet
Water is cold and wet; and
Earth is cold and dry.
Two of these elements (water and earth) are heavy, and seek their natural place at the centre of cosmos. Two of these elements are light and naturally rise away from the centre of the cosmos (fire quickly, air more slowly).
The resulting four elements are very closely associated with the four humours of Galenic medicine.
Alchemists, inquiring more closely into the nature of the four elements, generated alternative explanations of the fundamental constituents of matter, including the Paracelsian tria prima.
Further reading. G.E.R. Lloyd, Aristotle: The Growth and Structure of his Thought (Cambridge, 1968), ch. 8: 'The Physics of the Sublunary Region', esp. pp. 162-70.