Does "it is true" and "is quite customary" refer to "explanation"? and what does it mean? does it mean the "explanation" is both "true" and "customary"?
If, nevertheless, the foundation of this act, deduced by me on the ground of an enlarged conception of space, should be denied, only one other kind of explanation would remain, arising from a moral code of consideration that at present, it is true, is quite customary. This explanation would consist in the presumption that I myself and the honourable men and citizens of Leipzig, in whose presence several of these cords were sealed, were either common impostors, or were not in possession of our sound senses sufficient to perceive if Mr. Slade himself, before the cords were sealed, had tied them in knots. The discussion, however, of such a hypothesis would no longer belong to the dominion of science, but would fall under the category of social decency."