"Without symmetry and proportion there can be no principles in the design of any temple."
Vitruvius, Ten Books on Architecture (III.1.1)
Having maintained the engines of war as a military architect for Julius Caesar, Vitruvius dedicated De Architectura to Augustus, his heir, in hope that it would guide the emperor in the rebuilding of Rome. Although the tenets expounded in this treatise, the only one of its kind to survive, were largely ignored, they do preserve the principles of several dozen earlier Greek architects, including Chersiphron and his son, the architects of the Temple of Artemis at Ephesus, one of only four temples, which, by virtue of the "great excellence of and the wisdom of their conception they owe their place of esteem in the ceremonial worship of the gods."
It is in Books III and IV that Vitruvius promulgates his aesthetic. Derived from Hermogenes, the architect of the temple of Artemis at Magnesia, these rules on symmetry and proportion define what he calls eustyle (from eu stylos, literally "good column"), an architectural ideal based on the Ionic order as it was developed in Ionia on the Aegean coast of Asia Minor (Anatolia) in the mid-sixth century BC. (In the fifth century, the order appears in Greece, itself, where the best example is the Erechtheum on the Acropolis.)
Hermogenes prescribed a series of proportional relationships for temple colonnades based on the diameter of the column as the module or unit of measure. Ideally, intercolumniation (the space between the columns) should be two-and-a-quarter times the thickness of the column, itself, and the height of the column nine-and-a-half times its diameter. Closely spaced columns, for example, should be taller and more narrow than those farther apart. It is this sense of ratios and relationships that Vitruvius has in mind when he admonishes the reader to remember that, "in the members of a temple there ought to be the greatest harmony in the symmetrical relations of the different parts to the general magnitude of the whole." Changing one element requires that others be adjusted as well.
References: Vitruvius: The Ten Books on Architecture (1960) translated by Morris Morgan (Dover Books); Vitruvius: Ten Books on Architecture (2001) edited by Ingrid D. Rowland and Thomas Noble Howe; Art in the Hellenistic Age (1986) by J. J. Pollitt. The illustration is taken from History of Art (1995) by H. W. Janson.
