The introduction of Teichmüller’s name affords me an opportunity for mentioning that my attention was not directed to his brilliant researches into various questions connected with Greek philosophy, and more particularly with the systems of Plato and Aristotle, until it was too late for me to profit by them in the present work. I allude more particularly to his _Studien zur Geschichte der Begriffe_ (Berlin, 1874), and to his recently published _Literarische Fehden im vierten Jahrhundert vor Chr._ (Breslau, 1881). The chief points of the former work are, that Plato was really a pantheist or monist, not, as is commonly believed and as I have myself taken for granted, a dualist; that, as a consequence of the suppression of individuality which characterises his system, he did not really accept or teach the doctrine of personal immortality, although he wished that the mass of the people should believe it; that Plato no more attributed a transcendent existence to his ideas than did Aristotle to his substantial forms; and that in putting an opposite interpretation on his old master’s theory, Aristotle is guilty of gross misrepresentation. The most important point of the _Literarische Fehden_ is that Aristotle published his _Ethics_ while Plato was still alive and engaged in the composition of his _Laws_, and that certain passages in the latter work, of which one relates to free-will and the other to the unity of virtue (861, A ff. and 962 ff.) were intended as a reply to Aristotle’s well-known criticisms on the Platonic theory of ethics.