At the beginning of his commentary on Psalm 36, Ambrose notes that “All Scripture is divine, whether natural, whether mystical, whether moral.” After explaining that all three of these traditional philosophical categories are found in the book of Psalms, he says that Psalm 36 is devoted to moral teaching. In effect, using the rhetorical process of “discovery” (heuresis, inventio), he discovers that the “hypothesis” is ethical and this becomes the key to interpreting the psalm. The search for the hypothesis or argument (subject) was deeply rooted in the rhetorical and exegetical tradition long before Ambrose – who knew it at least from Cicero. Philo, with whose writings Ambrose was well acquainted, had used it in determining whether passages of the Law were to be interpreted ethike or physike. Thus rhetorical categories and philosophical categories come together in the process of interpretation. Additional background or justification for the procedure comes from the idea of Moses having preceded all the philosophers, a notion modelled on or borrowed from Homeric exegesis. This paper will explore the stages by which the categories and tools of rhetoric and philosophy came to be combined in the search for meaning.