When the mind considers every portion of the whole business with this intention, then the topics which have been reserved, will come into use, which we have already spoken of; and certain arguments will be derived from them both separately and unitedly.  Part of which arguments will depend on what is probable, part on what is necessary; there will be added also to conjecture questions, testimony, reports.  All of which things each party ought to endeavour by a similar use of these rules to turn to the advantage of his own cause.  For it will be desirable to suggest suspicions from questions, from evidence, and from some report or other, in the same manner as they have been derived from the cause, or the person, or the action.

Wherefore those men appear to us to be mistaken who think that this kind of suspicion does not need any regular system, and so do those who think that it is better to give rules in a different manner about the whole method of conjectural argument.  For all conjecture must be derived from the same topics; for both the cause of every rumour and the truth of it will be found to arise from the things attributed to him who in his inquiry has made any particular statement, and to him who has done so in his evidence.  But in every cause a part of the arguments is joined to that cause alone which is expressed, and it is derived from it in such a manner that it cannot be very conveniently transferred from it to all other causes of the same kind; but part of it is more rambling, and adapted either to all causes of the same kind, or at all events to most of them.

XV.  These arguments then which can be transferred to many causes, we call common topics.  For a common topic either contains some amplification of a well understood thing,—­as if any one were desirous to show that a man who has murdered his father is worthy of the very extremity of punishment; and this topic is not to be used except when the cause has been proved and is being summed up;—­or of a doubtful matter which has some probable arguments which can be produced on the other side of the question also; as a man may say that it is right to put confidence in suspicions, and, on the contrary, that it is not right to put confidence in suspicions.  And a portion of the common topics is employed in indignation or in complaint, concerning which we have spoken already.  A part is used in urging any probable reason on either side.

But an oration is chiefly distinguished and made plain by a sparing introduction of common topics, and by giving the hearers actual information by some topics, and by confirming previously used arguments in the same way.  For it is allowable to say something common when any topic peculiar to the cause is introduced with care; and when the mind of the hearer is refreshed so as to be inclined to attend to what follows, or is reawakened by everything which has been already said.  For all the embellishments of elocution, in which there is a great deal both of sweetness and gravity, and all things, too, which have any dignity in the invention of words or sentences, are bestowed upon common topics.