Complex sentence: This system was originally credited by Byzantine historians like George Ostrogorsky to Heraclius, where he provides Theophanes as a source, quoting the expression "the district of the themes" in the year 622 that shows the themes had begun to be created at this time.
Split into simpler sentences: This system was originally credited by Byzantine historians like George Ostrogorsky to Heraclius. He provides Theophanes as a source, quoting the expression "the district of the themes" in the year 622, showing the themes had begun to be created at this time.