This article explores the implications of a neglected aspect of that letter to the owner of the Primavera in which Marsilio Ficino pictures the human soul as mirroring the cosmos. The lunar metaphor adopted for the ratio mobilis, middle term of the soul, is found to be an integral feature of his ontology, developed primarily in relation to Aristotelian cosmology. As template for the metaphor, the division of perceptible reality into supralunary and sublunary contrasting halves can be seen to govern the imagery of the Primavera, the contrasting halves of which are centred on a figure with a lunar amulet and inescapable overall resemblance to a lunar deity described in a recently printed and highly popular classical work. Botticelli's painting embodies a metaphysical image of the cosmos reflected in the soul, as outlined in the letter, with the planetary deity Luna as the middle term of both, mediating between the celestial and terrestrial realms of the one and – as ratio– the intellectual and sensual realms of the other. The genre of the Primavera has been obscured. It is a great Quattrocento imago mundi.