The work of Cherniss on Aristotle's criticism of the Presocratics may be compared with that of Jaeger on the development of Aristotle's own thought as contained in his Aristoteles of 1923. Jaeger modestly described that epoch-making work as a Grundlegung or foundation for the history of the philosopher's development, and as such it has been of value not only for itself but in the stimulus it has given to further study, in the course of which the balance of its conclusions has been to some extent altered. Cherniss's own study is of the same pioneer kind, and if I confess to a feeling that it goes rather too far, the comparison with the now classic work of Jaeger will, I hope, make clear my general admiration and appreciation of the fact that it is a permanent contribution with which all future scholarship will have to reckon.I cannot at this stage even begin to discuss in detail the mass of erudition on which Cherniss's case is built up. Nevertheless, the very widespread acceptance of his strictures on Aristotle's historical sense suggest that anyone to whom they seem extreme should lose no time in giving voice to his misgivings, even in general terms, before they become irrevocably canonical. This thought has been prompted by the recent monograph of Mr. J. B. McDiarmid, Theophrastus on the Presocratic Causes, at the beginning of which we read simply that ‘the question of Aristotle's bias has been dealt with exhaustively by H. Cherniss’, whose views then become, without further remark, the starting-point of the younger scholar's own inquiry into the reliability of Theophrastus. Since in what follows I may speak critically of McDiarmid on several points, let me say that his main thesis, the dependence of Theophrastus on Aristotle in much of his φυσικων δόξαι and the consequent danger of regarding him as a separate authority for Presocratic thought, seems true enough. The derivation of Theophrastus's judgments from those of his master was already beginning to be recognised with fruitful results, and the time was ripe for a general review of the evidence.