There has recently been considerable renewed interest in Book 7 of the "Physics "of Aristotle, once regarded as merely an undeveloped forerunner to Book 8. The debate surrounding the importance of the text is not new to modern scholarship: for example, in the fourth century BC Eudemus, the Peripatetic philosopher associate of Aristotle, left it out of his treatment of the "Physics." Now, for the first time, Charles Hagen's lucid translation gives the English reader access to Simplicius' commentary on Book 7, an indispensable tool for the understanding of the text. Its particular interest lies in its explanation of how the chapters of Book 7 fit together and its reference to a more extensive second version of Aristotle's text than the one which survives today.