 The authors' view of embryology (Epistle 25) is also a collection from different sources. On the one hand, the Ikhwân were not physicians, though they display extensive knowledge of anatomy and share the Aristotelian thesis of the origin of the embryo from the sperm and menstrual blood, while they did not accept the ancient Hippocratic theory as developed by Galen, of a female sperm coexisting with that of the male in the formation of the human being. On the other hand, they emphasize an astronomical/astrological approach to embryology that was not in Greek sources. It is considered to be a new idea introduced by Islamic scientists, from whom it passed into the Latin world. Greek sources for Epistle 37 on love also vary.