This is the first critical edition of Epistles 15-21 of the Brethren of Purity, which explore the natural sciences and correspond to Aristotle's great works on philosophy of nature. Along with Epistle 22, "On Animals," Epistles 15-21 correspond to the corpus of Aristotle's great works on the philosophy of nature: Physica, De caelo, De generatione et corruption, and Meteorologica I-III. Meteorologica IV may correspond to Epistle 19 "On Minerals" (though...