'devoid of factual, empirical content'

Sunday reflection from Carl Hempel:

“The nature of the peculiar certainty of mathematics is now clear: A mathematical theorem is certain relatively to the set of postulates from which it is derived; i.e., it is necessarily true if those postulates are true; and this is so because the theorem, if rigorously proved, simply re-asserts part of what has been stipulated in the postulates… A mathematical truth is irrefutably certain just because it is devoid of factual, or empirical content.”