Aristotle distinguishes dialectics and philosophy with regard to their reach, and he distinguishes both over and against sophistry with regard to the way in which they comport themselves to the content of their speech: the sophist on one side and the philosopher and dialectician on the other. In opposition to the sophist, the dialectician and the philosopher are determined by the fact that they take that about which they speak seriously, they intend their speech to bring about an understanding of the content, whereas the sophist pays no attention to the substantive content of his speech but is simply concerned with the speech itself, its apparent reasonableness and its brilliance. Therefore the idea guiding the sophist is παιδεία, a certain education with regard to speaking about all things. This παιδεία characterizes the form, in the sense of being able to speak well, εὗ, about everything. Even Aristotle knows of this ideal of education in the sense of scientific training, and even with him, in a certain respect it refers to the form: i.e., παιδεία is not limited to a determinate realm of objects. Yet, with Aristotle, παιδεία means education with regard to the possibility of one's speech measuring up to the matter spoken about in each case, thus precisely the opposite of what the sophist means by παιδεία, namely education in the sense of an utter unconcern with substantive content, an unconcern that is, in fact, one of principle. For Aristotle, to be educated means that the person's speech measures up to precisely the content, to what is spoken about in each case. Since there are contents in many regions, παιδεία. cannot be characterized simply in terms of content. It concerns, rather, a determinate kind of training, the methodical attainment of the scientific level in questioning and research. Through this delimitation, sophistry is at the same time brought into connection with ἀληθεύειν, the disclosure of beings, which is what defines philosophy itself.
I will not pursue the historical conditions and will not present a historical characterization of sophistry. For that, you should consult Diels, Fragmente der Vorsokrater II. The main genuine source is Plato himself. Therefore a discussion about the historical situation of sophistry, given the prejudices of Plato, presents certain difficulties. Our consideration will proceed in a different direction, not toward sophistry in its cultural significance but toward understanding, from the idea of sophistry itself, that with which the sophist as sophist is involved: semblance, the false, the not, and negation.
b) Critique of the traditional interpretation of sophistry.
The interpretation of sophistry, as it developed historiographically, and in the usual history of philosophy, took the sophists as exponents of definite philosophical positions as regards knowledge and life, so that the sophists were considered skeptics, relativists, and subjectivists, whatever these terms might mean.