Aristotelian science seeks to define the essential nature of a thing and then to demonstrate the features the thing must have because of that nature. A philosophically inevitable question thus arises for Aristotelians: what is a nature? Is it a reality over and above (or perhaps “in”) the things whose nature it is? Is it a mental construction, existing only in our understanding of things; if so, on what basis is it constructed? This is the medieval problem of universals, or at least one way of thinking about the problem. In a classic formulation, Boethius states the problem in terms of the reality of genera and species, two main types of universals involved in an Aristotelian definition of essential nature (as in “a human being is a reasoning/ speaking animal,” which places us in the genus of animals and marks off our species by reference to our “difference” from other animals in reasoning or using language): “Plato thinks that genera and species and the rest are not only understood as universals, but also exist and subsist apart from bodies. Aristotle, however, thinks that they are understood as incorporeal and universal, but subsist in sensibles.” A rigorous tradition of, mainly Aristotelian, discussion originates from Boethius’s tentative exploration of the problem thus stated. But a more Platonic solution had been put into play about a century before Boethius by Augustine, and this, too, would have a rich development.