Such a study proceeds on the assumption that all religions are products of thought, commenced and continued in accordance with the laws of the human mind, and, therefore, comprehensible to the extent to which these laws are known. No one disputes this, except in reference to his own religion. This, he is apt to assert, had something “supernatural” about its origin. If this word be correctly used, it may stand without cavil. The “natural” is that of which we know in whole or in part the laws; the “supernatural” means that of which we do not at present know in any degree the laws. The domain of the supernatural diminishes in the ratio of the increase of knowledge; and the inference that it also is absolutely under the control of law, is not only allowable but obligatory. A second assumption must be that there is a unity of kind and purpose in all religions. Without this, no common law can exist for them. Such a law must hold good in all ages, in every condition of society, and in each instance. Hence those who explain religious systems as forms of government, or as systems of ethics, or as misconceived history, or as theories of natural philosophy, must be prepared to make their view good when it is universally applied, or else renounce the possibility of a Science of Religion; while those who would except their own system from what they grant is the law of all others, violate the principles of investigation and thereby the canons of truth. The methods of science are everywhere alike. Has the naturalist to explain an organism, he begins with its elements or proximate principles as obtained by analysis; he thence passes to the tissues and fluids which compose its members; these he considers first in a state of repose, their structure and their connections; then he examines their functions, the laws of their growth and action; and finally he has recourse to the doctrine of relations, _la théorie des milieux_, to define the conditions of its existence. Were such a method applied to a religion, it would lead us first to study its psychological elements, then the various expressions in word and act to which these give occasion, next the record of its growth and decay, and finally from these to gather the circumstantials of human life and culture which led to its historic existence. Some have urged that such a method should not be summoned to questions in mental philosophy. To do so, say they, is to confound things distinct, requiring distinct plans of study. Such a criticism might have had weight in the days when the mind was supposed to inhabit the body as a tenant a house, and have no relation to it other than that of a casual occupant. But that opinion is antiquated. More than three-fourths of a century ago the far-seeing thinker, Wilhelm von Humboldt, laid down the maxim that the phenomena of mind and matter obey laws identical in kind;[6-1] and a recent historian of science sums up the result of the latest research in these words: