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ISHOP BUTLER has well remarked that "probability is the guide of life," and, assuredly, if it be our guide in all practical concerns, in a still more significant sense may it be claimed as the source of the greater part of human knowledge. Indeed, in a rapid survey of the field of astronomy we are tempted to affirm, not merely that the theory of probability is of the utmost service to us, but that it is almost our sole method of discovering the truth. This will not seem a paradox to any one who will reflect that there is hardly an astronomical doctrine, even of the most elementary kind, of which it might not be said that our belief in it depends simply on the fact that its truth is, in a high degree, more probable than its falsehood. To those who are accustomed to apply the doctrine of probabilities habitually and, indeed, almost unconsciously, it affords the readiest touchstone by which many fallacious scientific notions can be dissipated. Let me give an illustration of what I mean. In the first book about astronomy which I read in my boyhood there was a glowing description of an investigation