Ptolemy did not write the Tetrabiblos to catalog superstition. He wrote it to give astrology the same kind of grounding he had just given astronomy in the Almagest, a physical mechanism rather than a bare assertion that the stars matter. The opening book argues that the sun visibly governs the seasons and the moon visibly governs tides and moisture, so it is reasonable, not mystical, to suppose the other planets exert comparable physical influence on terrestrial life, simply weaker and harder to isolate. That argument sets the tone for the whole work: astrology as an extension of natural philosophy, not a separate occult system running alongside it.
The four books divide the subject by scale rather than by technique. The first sets out the tools, the planets, signs, aspects and dignities, and their supposed physical qualities of heat, cold, moisture and dryness. The second turns to general or mundane astrology, the fate of nations, cities, climates and weather, before the third and fourth narrow to the individual nativity, the personal horoscope cast from the moment of birth, covering length of life, physical constitution, character, fortune and death in turn. That ordering, world before person, was itself influential: later astrological writing largely inherited Ptolemy's sense that a personal chart should be read against inherited background patterns rather than in isolation.
Ptolemy is notably cautious throughout about what astrology can and cannot promise, repeatedly distinguishing it from divination and insisting its predictions describe tendencies shaped by physical causes rather than fixed, unavoidable fates, a qualification later astrologers did not always preserve.