In order to delimit the field of theoretical biology, the author distinguishes in empirical biology a substructure and a superstructure. Empirical biology cannot be constituted without a minimum of reference to philosophical ideas such as the principle of identity (substructure); having regard to its objective, which is explanation by means of physico-chemical models, it does not easily avoid ontological aspirations. Further, experimental research makes great use of scientific theories. Finally, the elaboration of the empirical data of biology may find its fulfilment in an ontological science, philosophical biology, which is a branch of the philosophy of nature. Philosophical biology, superstructure of experimental biology, is a discipline intermediate between the special sciences of life, defined in an empirical sense, and metaphysics.