Consider upon what causes historical events depend:  the customs, character, and opinions of all the people concerned; the organisation of their government, and the character of their religious institutions; the development of industry among them, of the military art, of fine art, literature and science; their relations, commercial, political and social, with other nations; the physical conditions of climate and geographical position amidst which they live.  Hardly an event of importance occurs in any nation that is not, directly or indirectly, influenced by every one of these circumstances, and that does not react upon them.  Now, from the nature of the Canons of direct Induction, a satisfactory employment of them in such a complex and tangled situation as history presents, is rarely possible; for they all require the actual or virtual isolation of the phenomenon under investigation.  They also require the greatest attainable immediacy of connection between cause and effect; whereas the causes of social events may accumulate during hundreds of years.  In collecting empirical laws from history, therefore, only very rough inductions can be hoped for, and we may have to be content with simple enumeration.  Hence the importance of supporting such laws by deduction from the nature of the case, however faint a probability of the asserted connection is thereby raised; and this even if each law is valued merely for its own sake.  Still more, if anything worth the name of Historical Science is to be constructed, must a mere collection of such empiricisms fail to content us; and the only way to give them a scientific character is to show deductively their common dependence upon various combinations of the same causes.  Yet even those who profess to employ the Historical Method often omit the deductive half of it; and of course ‘practical politicians’ boast of their entire contentment with what they call ‘the facts.’

Sometimes, however, politicians, venturing upon deductive reasoning, have fallen into the opposite error of omitting to test their results by any comparison with the facts:  arguing from certain ‘Rights of Man,’ or ‘Interests of Classes,’ or ‘Laws of Supply and Demand,’ that this or that event will happen, or ought to happen, without troubling themselves to observe whether it does happen or ever has happened.  This method of Deduction without any empirical verification, is called by Mill the Geometrical; and, plainly, it can be trustworthy only where there is no actual conflict of forces to be considered.  In pure mathematical reasoning about space, time, and number, provided the premises and the reasoning be correct, verification by a comparison with the facts may be needless, because there is no possibility of counteraction.  But when we deal with actual causes, no computation of their effects can be relied upon without comparing our conclusions with the facts:  not even in Astronomy and Physics, least of all in Politics.