 His underlying “philosophy” is the view that there are real structures in the world which are “pictured” or represented, in an approximate fashion, by mathematical structures, symbolically \(\boldsymbol{PT} = \boldsymbol{W} (-) \boldsymbol{MT}\). The mathematical theory \(\boldsymbol{MT}\) used in a physical theory \(\boldsymbol{PT}\) contains as its core a “species of structure” \(\Sigma\). This is a meta-mathematical concept of Bourbaki which Ludwig introduced into the structuralistic approach. The contact between \(\boldsymbol{MT}\) to some “domain of reality” \(\boldsymbol{W}\) is achieved by a set of correspondence principles \((-)\), which give rules for translating physical facts into certain mathematical statements called “observational reports”. These facts are either directly observable or given by means of other physical theories, called “pre-theories” of \(\boldsymbol{PT}\). In this way a part \(\boldsymbol{G}\) of \(\boldsymbol{W}\), called “basic domain” is constructed. But it remains a task of the theory to construct the full domain of reality \(\boldsymbol{W}\), that is, the more complete description of the basic domain that also uses \(\boldsymbol{PT}\)-theoretical terms.