Chapter 11
That there should then be forms, or one certain thing besides the many, is not necessary, to the existence of demonstration, but it is necessary truly to predicate one thing of the many, for there will not be the universal unless this be so, and if there be not an universal, there will not be a medium, so that neither will there be a demonstration. It is essential then that there should be one and the same thing, which is not equivocal in respect of many: no demonstration however assumes that it is impossible to affirm and deny the same thing at one and the same time, unless it is requisite also thus to demonstrate the conclusion. It is demonstrated however by assuming the first to be true of the middle, and that it is not true to deny it, but it makes no difference
