CHRISTMAS, the season of Marley's ghost and Gabriel Grub and a host of other such stories, is an appropriate time to talk about ghosts. And we can again talk seriously and sincerely about ghosts and about the Thing, for, as Mr. Heard will show, the Victorian rationalism and purblind science that killed Marley and the goblins has been itself killed by a greater knowledge that brings us again to the threshold of the unseen. Mr. Heard will be remembered as a protagonist in some of the most stimulating broadcast discussions and debates, and he is the author of an interesting book of philosophy— ' Focus '—which will shortly appear.