" You are the most dogmatic man of my acquaintance!" In the course of a heated discussion, not very long ago, this remark was addressed by a distinguished medical man apparently to his opponent. Probably, the words were really aimed at the other members of the Society, with a view to discredit his antagonist, whose argument, possibly from inability, the speaker did not attempt to refute. Plainly the remark is a typical example of the argumentum ad hominem, an instance of that material fallacy, recognized by all logicians from Aristotle down to Archbishop Whately, as the irrelevant conclusion. Technically this fallacy is called ignoratio elenchi, or literally ignorance of the refutation, and the name indicates clearly its use in sophistry. It is the great resource of those that have to support a weak case. Some lawyers are familiar with it, as in the case of the English attorney for the defendant,