The most general division of the writings of Plato, is into those of the Sceptical kind, and those of the Dogmatical. In the former sort, nothing is expressly either proved or asserted : some philosophical question only is considered and examined ; and the reader is left to himself to draw such conclusions, and discover such truths, as the philosopher means to insinuate. This is done, either in the way of inquiry, or in the way of controversy and dispute. In the way of controversy are carried on all such dialogues, as tend to eradicate false opinions; and that, either indirectly, by involving them in difficulties, and embarrassing the maintainers of them ; or directly, by confuting them. In the way of inquiry proceed those, whose tendency is to raise in the mind right opinions; and that, either by exciting to the pursuit of some part of wisdom, and showing in what manner to investigate it; or by leading the way, and helping the mind forward in the search. And this is effected by a process through opposing arguments.
The dialogues of the other kind, the Dogmatical or Didactic, teach explicitly some point of doctrine: and this they do, either by laying it down in the authoritative way, or by proving it in the way of reason and argument. In the authoritative way the doctrine is delivered, sometimes by the speaker himself magisterially, at other times as de rived to him by tradition from wise men. The argumentative or demonstrative method of teaching, used by Plato, proceeds in all the dialectic ways, dividing, defining, demonstrating, and analysing; and the object of it consists in exploring truth alone.
From "General Introduction" to The Works of Plato, viz. His Fifty-Five Dialogues and Twelve Epistles, Floyer Sydenham and Thomas Taylor, eds. (London: 1804), page xciv. (The Introduction overall is Taylor's, but the passage and scheme is Sydenham's and is also found in his Synopsis of Plato; the Synopsis was a major influence on Samuel Taylor Coleridge.) So the scheme is:
Sceptical (insinuate ideas and leave it to the reader to draw conclusions)
(1) in the way of inquiry (directed to raising right opinion)
(a) by exciting to the pursuit of wisdom
(b) by helping in the investigation
(2) in the way of controversy and dispute (directed to eradicating false opinion)
(a) indirectly: showing embarrassing difficulties in false opinion
(b) directly: refuting false opinion
Dogmatical/Didactic (explicitly draw conclusions)
(1) by laying it down authoritatively
(a) magisterially, from the speaker's own authority
(b) traditionally, from the tradition of wise men
(2) by proving it with argument