On the basis of previous studies in relational biology and the phenomenological calculus, in my contribution I outline the mathematical foundations of biological perception generally, and visual perception specifically. In this approach, the premise is that objects in nature are not directly accessible, and that real manifestations are projections of these invariant objects. The morphology of observables is mathematically entailed by the duality of projections and projectors in a bilinear algebra that is the phenomenological calculus. The relationships between what is (...) not directly accessible (which can be defined as noumena) and phenomena may be explicated in terms of metabolism and repair, whence a geometric theory of the perceived, the visible in particular, may be formulated in the category-theoretic language of relational biology. (shrink)