Chalmers frowned. «I’ll get to that, if you give me time. As I see it, the method consists of filling your mind with the fundamental assumptions of the world in question. Now, what are the fundamental assumptions of our world? Obviously, those of scientific logic.»

«Such as —» said Shea.

«Oh, the principle of dependence, for instance. ‘Any circumstance in which alone a case of the presence of a given phenomenon differs from the case of its absence is casually relevant to that phenomenon.’»

«Ouch!» said Shea. «That’s almost as bad as Frege’s definition of number.»

Bayard droned: «The number of things in a given class —»

«Stop it, Walter! It drives me nuts!»

«— is the class of all classes that are similar to the given class.»

«Hrrm,» remarked Chalmers. «If you gentlemen are through with your joke, I’ll go on. If one of these infinite other worlds — which up to now may be said to exist in a logical but not in an empirical sense — is governed by magic, you might expect to find a principle like that of dependence invalid, but principles of magic, such as the Law of Similarity, valid.»

«What’s the Law of Similarity?» asked Bayard sharply.

«The Law of Similarity may be stated thus: Effects resemble causes. It’s not valid for us, but primitive peoples firmly believe it. For instance, they think you can make it rain by pouring water on the ground with appropriate mumbo jumbo.»

«I didn’t know you could have fixed principles of magic,» commented Shea.

«Certainly,» replied Chalmers solemnly. «Medicine men don’t merely go through hocus-pocus. They believe they are working through natural laws. In a world where everyone firmly believed in these Laws, that is, in one where all minds were attuned to receive the proper impressions, the laws of magic would conceivably work, as one hears of witch-doctors’ spells working in Africa today. Frazer and Seabrook have worked out some of these magical laws. Another is the Law of Contagion: Things once in contact continue to interact from a distance after separation. As you —»