In his major work, the Periphyseon, the ninth century Latin philosopher John Scottus Eriugena gives, with the help of what he calls "dialectic", a rational analysis of reality. According to him, dialectic is a science which pertains both to language and reality. Eriugena grounds this position in a realist ontological exegesis of the Aristotelian categories, which are conceived as categories of being. His interpretation tends to transform logical patterns, such as Porphyry's Tree or the doctrine of the categories, into a structure which is both ontological and logical, and to use them as tools for the analysis of the sensible world. The combination of dialectic interpreted as a science of being, capable of expressing truths about the sensible world as well as about discourse, with an ontological interpretation of logical concepts allows Eriugena to develop his metaphysical theory, a strong realism. Eriugena not only supports a theological realism (of divine ideas), but also, and principally, an ontological realism, the assertion of the immanent existence of forms. Eriugena claims that genera and species really subsist in the individuals: they are completely and simultaneously present in each of the entities which belong to them.