2.  First of all I have to urge, what is of most importance in such matters, our own custom, which has the force of law, because the rules have been handed down to us by holy men.  It is as follows:  if any one, overcome by impurity, falls into unlawful intercourse with two sisters, this is not to be looked upon as marriage, nor are they to be admitted at all into the Church until they have separated from one another.  Wherefore, although it were possible to say nothing further, the custom would be quite enough to safeguard what is right.  But, since the writer of the letter has endeavoured to introduce this mischief into our practice by a false argument, I am under the necessity of not omitting the aid of reasoning; although in matters which are perfectly plain every man’s instinctive sentiment is stronger than argument.