This fallacy underlies much that is being urged in favour of a common origin for religious doctrines and methods of worship. A single source of religious belief or of religious phenomena is preferred to several sources as being more tidy and more in keeping with what we have learnt to expect in other departments of research. It may be illogical, but still it is recommended as a safe guide to the truth. Indeed, it is difficult for a modern student to conceive how any real advance can be made in scientific pursuits unless the principle, which prefers one explanation of phenomena to many, is favoured. Before the days of Kepler and of Newton it may have been possible, it may be possible still, to imagine more than one explanation of the fall of a heavy body to the ground and of the action of one inert mass upon another. The law of gravity, as elaborated by Newton, represents what, so far as we know, has invariably happened and what we believe will invariably happen in space between two or more bodies, namely, that they will, as heretofore, each attract all the other bodies directly as their mass and inversely as the square of their distance. This law is not merely preferred before all other laws; it is the very foundation of the whole of what is called Physical Astronomy. It is a law to which there are, within its own province, no known exceptions. We accept this law not because we prefer one explanation to many, but because it meets not only the requirements of cases which might conceivably be explained in other ways but also the requirements of cases for which no other explanation has been suggested or conceived. Among laws, which are not received as self-evident, the law of gravity is unique. This will be clear to anyone who contrasts the secure position which it occupies with the perilous position occupied by laws which have been formulated within recent years. Men do not prefer Newton’s explanation to other explanations: the evidence in its favour is so overwhelming that they feel compelled to accept it. It is far otherwise with other laws like evolution. These fascinate or repel from the very first. Preference undoubtedly enters into the complex intellectual process which leads us first to accept and then to defend this or that explanation of an array of facts. And this preference, admittedly illogical, may arise from our limited knowledge of the facts or from regard for some particular protagonist of one of many conflicting theories; but, other things being equal, it seizes hold of that explanation which claims to cover the most ground and to reconcile the largest number of facts. It only becomes mischievous when it claims infallibility.