Now while it is true that in order to understand the present we must know the past, and that because the present is a product of the past, it is also true that a condition of understanding is to interpret the past by the present. In ordinary affairs this is not questioned. When geologists set out to explain the causes of changes in the earth's surface, they utilize the present-day knowledge of existing forces, and by prolonging their action backward explain the features of the period they are studying. When historians seek to explain the conduct of, say Henry the Eighth, they take their knowledge of the motives animating existing human nature, and by placing that in a sixteenth century setting manage to present us with a picture of the period. So, again, when the thirteenth century monkish historian gravely informs us that a particular epidemic was due to the anger of God against the wickedness of the people, we put that interpretation on one side and use our own knowledge to find in defective social and sanitary conditions the cause of what occurred. Illustrations to the same end may be found in every direction. It is, indeed, not something that one may accept or reject as one may take or leave a political theory, it is an indispensible condition of rational thinking on any subject whatsoever. Accepted everywhere else, it is in connection with religion that one finds this principle, not openly challenged, for there are degrees of absurdity to which even the most ardent religionist dare not go, but it is quietly set on one side and a method adopted which is its practical negation. Either the procedure is inverted and the present is interpreted by the past, as when it is assumed that because God did certain things in the past therefore he will continue to do the same things in the present, or it is assumed that the past was unlike the present, and, therefore, the same method of interpretation cannot be applied to both cases. Both plans have the effect of landing us, if not in lunacy, at least well on the way to it.