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The Anthologies of the second-century astrologer Vettius Valens (120-c.175 CE) is the most extensive surviving practical astrological text from the period. Despite this, the theoretical underpinnings of the Anthologies have been understudied; in general, the work has been overshadowed by Ptolemy's contemporaneous Tetrabiblos. While the Tetrabiblos explicitly aims to present a systematic account of astrology, Valens' work is often characterised as a miscellaneous collection, of interest to historians only for the evidence it preserves about the practical methods used in casting horoscopes. In this article, we argue that the Anthologies is also an invaluable resource for engagement with the conceptual basis of astrology. As a case study, we take a section of Anthologies Book 1 which lists the possible astrological effects of planets, both alone and in 'combinations' of two and three. We demonstrate that analysing Valens' descriptions quantitatively with textual analysis reveals a consistent internal logic of planetary combination. By classifying descriptive terms as positive or negative, we show that the resulting 'sentiment' of planetary combinations is well-correlated with their component parts. Furthermore, we find that the sentiment of three-planet combinations is more strongly correlated with the average sentiment of their three possible component pairs than with the average sentiment of individual planets, suggesting an iterative combinatorial logic. Recognition of this feature of astrological practice has been neglected compared to the mathematical methods for calculating horoscopes. We argue that this analysis not only provides evidence that the astrological lore detailed in Valens is more consistent than is often assumed, but is also indicative of a wider methodological technique in practical astrology: combinatorial reasoning from existing astrological lore.