Even at a dinner or a card-table, each of them had to be certain that, in the diner or partner who sat opposite to him, there was, latent and not yet made use of, a certain brand of knowledge which would enable him to identify the rubbish with which so many houses in Paris were littered as genuine mediaeval or renaissance "pieces" and, whatever the subject of discussion, to apply the critical standards common to all their party whereby they distinguished good work from bad.