Enlightenment deism consisted of two philosophical assertions: (a) reason, along with features of the natural world, is a valid source of religious knowledge, and (b) revelation is not a valid source of religious knowledge. Different deist authors expanded on these two assertions to create what Leslie Stephen later termed the "constructive" and "critical" aspects of deism. "Constructive" assertions— assertions that deist writers felt were justified by appeals to reason and features of the natural world (or perhaps were intuitively obvious) — included: