For some decades after the formulation of General Relativity, the attitude towards Lorentzian geometry was mostly dedicated to understanding its physical implications and not concerned with theoretical issues. However, early attempts to use causality as a starting point have been provided by Weyl and Lorentz. Alfred Robb in two books in 1914 and 1936 suggested an axiomatic framework where the causal precedence played a critical role. The first explicit proposal of quantising the causal structure of spacetime is attributed by S. Surya to Kronheimer and Penrose, who invented Causal spaces in order to admit structures which can be very different from a manifold. Causal spaces are defined axiomatically, by considering not only causal precedence, but also chronological precedence.