 The latter issue was taken up in Weyl’s “Habilitation” lecture (1910), where he addressed the general question: when is a relation explicitly definable from a set of given primitive concepts in mathematics? First of all, he considers, as a concrete case study, the problem of characterizing the explicitly definable concepts of elementary plane geometry: these can be inductively generated by means of five basic definition principles from two suitably chosen primitive concepts (e.g., the identity between points =, and the ternary relation \(E(a, b, c)\), ‘the distance of point \(a\) from point \(b\) is the same as the distance of point \(a\) from point \(c\)’).