standing until the 18th century, is thus: "even if it be not entirely infallible, at least its possibilities have appeared worthy of the highest regard". Having justified his intellectual involvement in the study, according to the philosophical principles of his day, Ptolemy then turns his attention to the practical theory of astrology, and the rationale that lies behind the arrangement of its principles. Introduction of principles One of the unique features of the Tetrabiblos, amongst the astrological texts of its period, is the extent to which the first book not only introduces the basic astrological principles, but synthesises and explains the reasoning behind their reported associations in line with Aristotelian philosophy. Chapter four, for example, explains the "power of the planets" through their associations with the creative humoral qualities of warmth or moisture, or the reductive qualities of cold and dryness. Hence Mars is described as a destructive planet because its humoral association is excessive dryness, whilst Jupiter is defined as temperate and fertilising because its association is moderate warmth and humidity. These associations are based on the arrangements of the planets with regard to the Sun, as perceived from the geocentric perspective, by which their orbits are measured is if they are centred upon the Earth. Uniting these Aristotelian principles with a prevalent Greek philosophy employed by Zeno of Citium and the Pythagoreans, the next three chapters arrange the planets into pairs of opposites. They may be benefic (moderately warming or moistening) or malefic (excessively cooling or drying); masculine (drying) or feminine (moistening); active and diurnal (suited to the qualities of the day and aligned with the nature of the Sun) or passive and nocturnal (suited to the qualities of the night and aligned with the nature of the Moon). Since these humoral associations derive from configurations with the Sun, chapter eight describes how they are subtly modified according to the phase of each planet's synodic cycle with the Sun. Chapter nine discusses the "power of the fixed stars". Here, rather than give direct humoral associations, Ptolemy describes their "temperatures" as being like that of the planets he has already defined. Hence Aldebaran ("called the Torch") is described as having "a temperature like that of Mars", whilst other stars in the Hyades are "like that of Saturn and moderately like that of Mercury". At the end of the chapter Ptolemy clarifies that these are not his proposals, but are