II.  As every careful method of arguing has two divisions,—­one of discovering, one of deciding,—­Aristotle was, as it appears to me, the chief discoverer of each.  But the Stoics also have devoted some pains to the latter, for they have diligently considered the methods of carrying on a discussion by that science which they call dialectics; but the art of discovering arguments, which is called topics, and which was more serviceable for practical use, and certainly prior in the order of nature, they have wholly disregarded.  But we, since both parts are of the greatest utility, and since we intend to examine each if we have time, will now begin with that which is naturally the first.

As therefore the discovery of those things which are hidden is easy, if the place where they are hidden is pointed out and clearly marked; so, when we wish to examine any argument, we ought to know the topics,—­for so they are called by Aristotle, being, as it were, seats from which arguments are derived.  Therefore we may give as a definition, that a topic is the seat of an argument, and that an argument is a reason which causes men to believe a thing which would otherwise be doubtful.  But of those topics in which arguments are contained, some dwell on that particular point which is the subject of discussion; some are derived from external circumstances.  When derived from the subject itself, they proceed at times from it taken as a whole, at times from its parts, at times from some sign, and at others from things which are disposed in some manner or other towards the subject under discussion; but those topics are derived from external circumstances which are at a distance and far removed from the same subject.

But a definition is employed with reference to the entire matter under discussion which unfolds the matter which is the subject of inquiry as if it had been previously enveloped in mystery.  The formula of that argument is of this sort:  “Civil law is equity established among men who belong to the same city, for the purpose of insuring each man in the possession of his property and rights:  and the knowledge of this equity is useful:  therefore the knowledge of civil law is useful.”  Then comes the enumeration of the parts, which is dealt with in this manner:  “If a slave has not been declared free either by the censor, or by the praetor’s rod, or by the will of his master, he is not free:  but none of those things is the case:  therefore he is not free.”  Then comes the sign; when some argument is derived from the meaning of a word, in this way:—­As the Aelian Sentian law orders an assiduus[63] to support an assiduus, it orders a rich man to support a rich man, for a rich man is an assiduus, called so, as Aelius says, from asse dando.

III.  Arguments are also derived from things which bear some kind of relation to that which is the object of discussion.  But this kind is distributed under many heads; for we call some connected with one another either by nature, or by their form, or by their resemblance to one another, or by their differences, or by their contrariety to one another, or by adjuncts, or by their antecedents, or by their consequents, or by what is opposed to each of them, or by causes, or by effects, or by a comparison with what is greater, or equal, or less.