Concerning philosophy in general, the doctrine of the Stoics was, that wisdom consists in the knowledge of things divine and human; that philosophy is such an exercise of the mind as produces wisdom; that in this exercise consists the nature of virtue; and consequently, that virtue is a term of extensive meaning, comprehending the right employment of the mind in reasoning, in the study of nature, and in morals. The wisdom of the Stoics is either progressive, through several stages; or perfect, when every weakness if subdued, and every error corrected, without the possibility of a relapse into folly, or vice, or of being again enslaved by any passion, or afflicted by any calamity. With Socrates and the Cynics, Zeno represented virtue as the only true wisdom; but being disposed to extend the pursuits of his wise man into the regions of speculation and science, he gave, after his usual manner, a new signification to an old term, and comprehended the exercise of the understanding in the search of truth, as well as the government of the appetites and passions, under the general term, virtue. The great importance of the united exercise of the intellectual and active powers of the mind, are thus beautifully asserted by the philosophical emperor:--"Let every one endeavor so to think and act, that his contemplative and active faculties may at the same time be going on towards perfection. His clear conceptions, and certain knowledge, will then produce within him an entire confidence in himself, unperceived perhaps by others, though not affectedly concealed, which will give a simplicity and dignity to his character; for he will at all times be able to judge, concerning the several objects which come before him, what is their real nature, what place they hold in the universe, how long they are by nature fitted to last, of what materials they are composed, by whom they may be possessed, and who is able to bestow them, or take them away." The sum of the definitions and rules given by the Stoics concerning logic is this:--Logic is either rhetorical or dialectic. Rhetorical logic is the art of reasoning and discoursing on those subjects which require a diffuse kind of declamation. Dialectic is the art of close argumentation in the form of disputation or dialogue. The former resembles an open, the latter, a closed hand.--Rhetoric is of three kinds, deliberative, judicial, and demonstrative. The dialectic art is the instrument of knowledge, as it enables a man to distinguish truth from error, and certainty from bare probability. This art considers things as expressed by words, and words themselves. External things are perceived by a certain impression, made either upon some parts of the brain, or upon the percipient faculty, which may be called an image, since it is impressed upon the mind, like the image of a seal upon wax.