Hence Socrates, in the Republic, speaking of the power of dialectic, says that it surrounds all disciplines like a defensive enclosure, and elevates those that use it to the good itself, and the first unities; that it purifies the eye of the soul; establishes itself in true beings, and, the one principle of all things, and ends at last in that which is no longer hypothetical. The power of dialectic, therefore, being thus great, and the ends of this path so mighty, it must by no means be confounded with arguments which are alone conversant with opinion: for the former is the guardian of sciences, and the passage to it is through these, but the latter is perfectly destitute of disciplinative science. To which we may add, that the method of reasoning which is founded in opinion, regards only that which is apparent; but the dialectic method endeavors to arrive at the one itself, always employing for this purpose steps of ascent, and at last beautifully ends in the nature of the good. Very different therefore is it from the merely logical method, which presides over the demonstrative phantasy, is of a secondary nature, and is alone pleased with contentious discussions. For the dialectic of Plato for the most part employs divisions and analyses as primary sciences, and as imitating the progression of beings from the one, and their conversion to it again. It likewise sometimes uses definitions and demonstrations, and prior to these the definitive method, and the divisive prior to this. On the contrary, the merely logical method, which is solely conversant with opinion, is deprived of the incontrovertible reasonings of demonstration.
The following is a specimen of the analytical method of Plato's dialectic. Of analysis there are three species. For one is an ascent from sensibles to the first intelligibles; a second is an ascent through things demonstrated and subdemonstrated, to undemonstrated and immediate propositions; and a third proceeds from hypothesis to unhypothetical principles. Of the first of these species, Plato has given a most admirable specimen in the speech of Diotima in the Banquet. For there he ascends from the beauty about bodies to the beauty in souls; from this to the beauty in right disciplines; from this again to the beauty in laws; from the beauty in laws to the ample sea of beauty (Greek: to polu pelagos tou kalou); and thus proceeding he at length arrives at the beautiful itself.