These considerations lead us to adopt the following criteria of genuineness: (1) That is most certainly Plato's which Aristotle attributes to him by name, which (2) is of considerable length, of (3) great excellence, and also (4) in harmony with the general spirit of the Platonic writings. But the testimony of Aristotle cannot always be distinguished from that of a later age (see above); and has various degrees of importance. Those writings which he cites without mentioning Plato, under their own names, e.g. the Hippias, the Funeral Oration, the Phaedo, etc., have an inferior degree of evidence in their favour. They may have been supposed by him to be the writings of another, although in the case of really great works, e.g. the Phaedo, this is not credible; those again which are quoted but not named, are still more defective in their external credentials. There may be also a possibility that Aristotle was mistaken, or may have confused the master and his scholars in the case of a short writing; but this is inconceivable about a more important work, e.g. the Laws, especially when we remember that he was living at Athens, and a frequenter of the groves of the Academy, during the last twenty years of Plato's life. Nor must we forget that in all his numerous citations from the Platonic writings he never attributes any passage found in the extant dialogues to any one but Plato. And lastly, we may remark that one or two great writings, such as the Parmenides and the Politicus, which are wholly devoid of Aristotelian (1) credentials may be fairly attributed to Plato, on the ground of (2) length, (3) excellence, and (4) accordance with the general spirit of his writings. Indeed the greater part of the evidence for the genuineness of ancient Greek authors may be summed up under two heads only: (1) excellence; and (2) uniformity of tradition--a kind of evidence, which though in many cases sufficient, is of inferior value.