Having thus divided the dialogues of Plato, in respect of that inward form or composition, which creates their genius; and again, with reference to that outward form, which marks them, like flowers and other vegetables, with a certain character; we are further to make a division of them, with regard to their subject and their design; beginning with their design, or end, because for the sake of this are all the subjects chosen. The end of all the writings of Plato is that, which is the end of all true philosophy or wisdom, the perfection and the happiness of man. Man therefore is the general subject; and the first business of philosophy must be to inquire what is that being called man, who is to be made happy; and what is his nature, in the perfection of which is placed his happiness. As however, in the preceding part of this Introduction, we have endeavored to give the outlines of Plato's doctrine concerning man, it is unnecessary in this place to say any thing further on that subject.
The dialogues of Plato, therefore, with respect to their subjects, may be divided into the speculative, the practical, and such as are of a mixed nature. The subjects of these last are either general, comprehending both the others; or differential, distinguishing them. The general subject are either fundamental, or final: those of the fundamental kind are philosophy, human nature, the soul of man; of the final kind are love, beauty, good. The differential regard knowledge, as it stands related to practice; in which are considered two questions: one of which is, whether virtue is to he taught; the other is, whether error in the will depends on error in the judgment. The subjects of the speculative dialogues relate either to words, or to things. Of the former sort are etymology, sophistry, rhetoric, poetry; of the latter sort are science, true being, the principles of mind, outward nature. The practical subjects relate either to private conduct, and the government of the mind over the whole man; or to his duty towards others in his several relations; or to the government of a civil state, and the public conduct of a whole people. Under these three heads rank in order the particular subjects practical; virtue in general, sanctity, temperance, fortitude, justice, friendship, patriotism, piety; the ruling mind in a civil government, the frame and order of a state, law in general, and lastly, those rules of government and of public conduct, the civil laws.