Mach was the first great modern relationalist. In his writing, the observable properties of bodies are considered to define physical states, and this pragmatism is taken far more seriously than the concept of universal (theoretical) law. Later, Mach discusses causality: "Where we assign a cause, we only express a relation of connection, an existing of fact; that is to say, we describe. ... It is far better to regard the conceptual determinative elements of a fact as dependent upon one another in exactly the same sense as the mathematician, for example the geometer, does."
This is very much in the spirit of relational set theory, the axioms of which are properly formulated in topos theory. In particular, the axiom of comprehension states that there exists a set (contained in a chosen universal set) containing all elements $x$ such that $x \in \phi$, where $\phi$ is a property that may be stated logically. Recall that it was Gray who first considered extending this axiom to category theory, whereby he found himself developing the Gray tensor product for bicategories.
 in English, ed. B. McGuinness (Reidel 1986)