Ptolemy's Almagest was an authoritative text on astronomy for more than a thousand years, and the Tetrabiblos, its companion volume, was equally influential in astrology, the study of the effects of astronomical cycles on earthly matters.

"I know that I am mortal, the creature of one day; but when I explore the winding courses of the stars I no longer touch with my feet the Earth: I am standing near Zeus himself, drinking my fill of Ambrosia, the food of the gods."

[citation needed] The intellectual standing of astrology collapsed rapidly at the end of the 17th century, but the historical impact of the Tetrabiblos upon world culture continues to engage the attention of scholars of classical philosophy and the history of sciences in antiquity.

[16] Ptolemy wrote at a time when "physics" was defined by philosophy, and his account of stellar effects was expressed in terms of the four Aristotelian qualities (hot, cold, wet, and dry) set against the philosophical notion of universal unity and cosmic harmony.

[14] Of this, Lynn Thorndike, in his History of Magic and Experimental Science, writes: "Only the opponents of astrology appear to have remained ignorant of the Tetrabiblos, continuing to make criticisms of the art which do not apply to Ptolemy's presentation of it or which had been specifically answered by him".

[21] The historical importance and influence of the Tetrabiblos is attested by the many ancient, Medieval and Renaissance commentaries published about it, as well as the many translations and paraphrased editions that aim to reproduce its content in an accessible manner.

[27] In this, and further introductory remarks, he reveals his view that astrological prediction is extremely difficult and easily subject to error, but satisfactorily attainable to those who possess the necessary skill and experience, and of too much benefit to be dismissed simply because it can sometimes be mistaken.

[34] Equally, Ptolemy's narrative was drawn upon by later philosophers and astronomers, such as Johannes Kepler, who used similar examples and the same order of arguments to explain the physical foundation of some astrological claims.

[38] He points out how the Sun has the greatest influence upon the Earth's seasonal and daily cycles, and that most things in nature are synchronised by the Moon: ... as the heavenly body nearest the Earth, the Moon bestows her effluence most abundantly upon mundane things, for most of them, animate or inanimate, are sympathetic to her and change in company with her; the rivers increase and diminish their streams with her light, the seas turn their own tides with her rising and setting, and plants and animals in whole or in some part wax and wane with her.

These bring about elemental changes (hot, windy, snowy or watery conditions, etc): led by the Sun, activated by the Moon and aided by the planetary configurations and the fixed stars' phenomena.

[48] He takes a balanced position in the argument of fate versus free will in writing that certain things, because their effective causes are numerous, become inevitable, whilst others are able to be averted by the act of astrological prediction.

[55] Chapter ten returns to the humoral theme more explicitly, clarifying that the zodiac is aligned to the seasons and so expressive of the shifting emphasis through moisture, warmth, dryness and cold, (as brought about by spring, summer, autumn, and winter).

[56] The remainder of book one (up to the final two chapters which concern the planetary phases and aspectual applications), presents the rulerships, divisions, and configurations of the zodiac signs, most of which are related to astronomical definitions, seasonal effects, physics and geometry.

[63] His approach finds elegant expression where he is demonstrating the logic of schematic arrangements (such as the philosophical principles behind the planetary rulership of signs),[64] but is noted to convey detachment in regard to the elements of astrology that are not so obviously plausible.

[67] On the other hand, the objective tone that marks his style; his assertion that the subject is natural (by which he makes no demand for oaths of secrecy from its students as some contemporaries do);[68] and the way he shows respectful reference to alternative views without vilifying authors whose practices might differ from his own,[69] all helped to secure the text's historical reputation as an intellectually superior one.

Classics scholar Mark Riley raised these points in his assessment that Ptolemy approached the subject of astrology with exactly the same theoretical inclination that he applied to astronomy, geography and the other sciences on which he wrote.

This offers a comprehensive review of ethnic stereotypes, eclipses, significations of comets and seasonal lunations, as used in the prediction of national economics, wars, epidemics, natural disasters and weather patterns.

Communities that live close to the equator, for example, are described as having black skins, small statures, and thick woolly hair, as a protective response to the burning heat of that location.

[76] Observed characteristics influence his categorisation of Britain as having a closer affinity with Aries and Mars (by which "for the most part its inhabitants are fiercer, more headstrong and bestial"), whilst Spain is reported to be more subject to Sagittarius and Jupiter, (from which is evidenced "their independence, simplicity and love of cleanliness").

[90] It is noted that here Ptolemy uses principles that fall outside the neat theoretical logic he presents in book I, being explicable only in terms of the mythological and omen tradition inherited from his ancient sources.

Jim Tester has commented on how Ptolemy goes into an unusual level of detail[102] in a responsibility that Bouché-Leclercq described as "the chief task of astrology, the operation judged most difficult by practitioners, most dangerous and damnable by its enemies".

Diseases of the soul are defined as "affections which are utterly disproportionate and as it were pathological"[110] including insanity, inability to exercise moderation or restraint, instability of the emotions, depraved sexuality, morbid perversions, and violent afflictions of the intellectual and passive parts of the mind.

His exploration of individual horoscopes continues into book IV, the only distinction of content being that subsequent topics relate to material matters and life experiences: what Ptolemy refers to as "external accidentals".

[111] Chapter 2, on material wealth, employs the "so-called 'Lot of Fortune'" although Ptolemy's instruction conflicts with that of many of his contemporaries in stating that for its calculation "we measure from the horoscope the distance from the sun to the moon, in both diurnal and nocturnal nativities".

[118]The information in the passage can be summarised as follows:[119] The book ends with a brief discussion of astronomical and symbolic cycles used in the prediction of timed events, which includes mention of (primary) directions, annual profections, ingresses, lunations and transits.

[123] Despite Ptolemy's prominence as a philosopher, the Dutch historian of science Eduard Jan Dijksterhuis criticizes the Tetrabiblos, stating that "it only remain puzzling that the very writer of the Almagest, who had taught how to develop astronomy from accurate observations and mathematical constructions, could put together such a system of superficial analogies and unfounded assertions.

[130] The content of the Paraphrase is close to that of manuscripts of the Tetrabiblos, but it uses simplified text with the aim of providing what Heilen calls "a more easily understandable version of the difficult original work".

The 'Translator's Preface' was critical of Robbins' understanding of some of the "conceptual issues involved" and argued the need for a new English translation which recognised the "probable superiority of the Teubner text edited by Boll and Boer in 1940".

This claimed to be based on a heavily corrupted manuscript which required numerous conjectures by a scholarly friend of Wolf, who preferred to remain anonymous rather than face reproaches for "dabbling in this sort of literature".

[138] Jerome Cardan was the first to declare the work a forgery based on such differences, referring in his commentary on the Tetrabiblos to an argument of Galen: "In the old days, kings who were trying to establish great libraries bought the books of famous men at very high prices.