Iamblichus's argument begins from the Platonic and Neoplatonic premise that the highest principle — the One or the Good — transcends all categories of thought, including the categories of being and intellect. This means that thinking about the One cannot reach it: every thought is a relation between thinker and thought, and the One transcends both terms of this relation. Philosophical dialectic, however refined, remains within the domain of intellect — it can approach the One asymptotically but cannot cross the final gap. What is needed is a mode of contact that operates below or above the level of thought.
Ritual practice operates through the objective connections between material and divine things — the sympathies and participations that run through the entire hierarchy of being. These connections do not depend on the practitioner's understanding of them to be effective. A stone that participates in the solar principle works its connection to the sun whether or not the person holding it can give a philosophical account of why it does so. This is not an anti-intellectual point but a metaphysical one: some real connections in the order of nature are more fundamental than the conceptual representations by which philosophy attempts to grasp them.
Iamblichus does not wish to abandon philosophy but to locate it correctly within a larger framework. Philosophy is indispensable for purifying the intellect, ordering the soul's ethical life, and understanding the structure of the divine hierarchy. But its deliverances about the nature of the highest principle — the One — are inherently provisional and inadequate. Theurgy completes what philosophy cannot: by establishing the practitioner in a state of genuine reception, it opens the soul to a contact with the divine that philosophy can describe but not itself produce. The philosopher who does not practise theurgy is like someone who knows a great deal about music but has never heard it.
The claim that theurgy is superior to dialectic was among the most controversial in De Mysteriis and was challenged by those in the Platonic tradition (including Porphyry) who held that pure philosophical contemplation was the highest spiritual path. Proclus later attempted a synthesis: theurgy and philosophy are complementary, each operating at its proper level. This synthesis became the dominant position of late Neoplatonism and influenced Renaissance Neoplatonism through Ficino's translation and commentary on Proclus.