اعلم أن أرسطاطاليس الحكيم دوّن هذا العلم بأمر الإسكندر الرومي
Know that Aristotle the philosopher codified this science by the command of Alexander the Roman.
Codification means collecting and arranging discussions into systematic principles and chapters. The textbook associates Aristotle's codification with Alexander's command; this is presented here as the book's historical account.
The educational point is that people reasoned before the science was formally organized, but organizing its rules made them easier to teach, examine and apply.