According to the earliest source of information , the library was initially organized by Demetrius of Phaleron , a student of Aristotle , under the reign of Ptolemy I Soter ( ca .367 BCâ '' ca .283 BC ) . According to the earliest source of information , the pseudepigraphic Letter of Aristeas , the library was initially organized by Demetrius of Phaleron , a student of Aristotle , under the reign of Ptolemy Soter ( ca .367 BCâ '' ca .283 BC ) .