Returning then to the pyramid, we see that in balancing the claims of chance and design we must, in fairness to the latter, reckon to its account several other values as well as that of π, e.g. sqrt{2} and sqrt{3}, and a few more such simple and familiar ratios, as well as some of their multiples. But though the number of such values which _might_ be reckoned, on the ground that they are actually known to us, is infinite, yet the number that _ought_ to be reckoned, on the ground that they could have been familiar to the builders of a pyramid, are very few. The order of probability for or against the existence of design will not therefore be seriously altered here by such considerations.[7] 18. Up to this point it will be observed that what we have been balancing against each other are two forms of agency,--of human agency, that is,--one acting through chance, and the other by direct design. In this case we know where we are, for we can thoroughly understand agency of this kind. The problem is indeed but seldom numerically soluble, and in most cases not soluble at all, but it is at any rate capable of being clearly stated. We know the kind of answer to be expected and the reasons which would serve to determine it, if they were attainable. The next stage in the enquiry would be that of balancing ordinary human chance agency against,--I will not call it direct spiritualist agency, for that would be narrowing the hypothesis unnecessarily,--but against all other possible causes. Some of the investigations of the Society for Psychical Research will furnish an admirable illustration of what is intended by this statement. There is a full discussion of these applications in a recent essay by Mr F. Y. Edgeworth;[8] but as his account of the matter is connected with other calculations and diagrams I can only quote it in part. But I am in substantial agreement with him. "It is recorded that 1833 guesses were made by a 'percipient' as to the suit of cards which the 'agent' had fixed upon. The number of successful guesses was 510, considerably above 458, the number which, as being the quarter of 1833, would, on the supposition of pure chance, be more likely than any other number. Now, by the Law of Error, we are able approximately to determine the probability of such an excess occurring by chance. It is equal to the extremity of the tail of a probability-curve such as [those we have already had occasion to examine].... The proportion of this extremity of the tail to the whole body is 0.003 to 1. That fraction, then, is the probability of a chance shot striking that extremity of the tail; the probability that, if the guessing were governed by pure chance, a number of successful guesses equal or greater than 510 would occur": odds, that is, of about 332 to 1 against such occurrence.