  had obtained through the theory of groups, and in particular through  the theory of group representations—as augmented by his own  contributions—what he felt was a proper mathematical  understanding of tensors, tensor symmetries, and the reason they  represent the source of all linear quantities that might arise in  mathematics or physics. Once again, he had come to appreciate the  importance of the theory of groups—and now especially the  theory of group representation—for gaining insight into  mathematical questions suggested by relativity theory. Unlike his  work on the space problem …Weyl now found himself drawing upon far more than the rudiments of group theory. … And of course   Cartan[87]  had showed that the space problem could also be resolved with the aid of results about representations. In short, the representation theory of groups had proved itself to be a powerful tool for answering the  sort of mathematical questions that grew out of Weyl’s involvement  with relativity theory.