We can, of course, see that in the environment of any nation or class at any given time there are some facts which constitute for all its members a common experience, and therefore a common influence. Climate is such a fact, or the discovery of America, or the invention of printing, or the rates of wages and prices. All nonconformists are influenced by their memory of certain facts of which very few churchmen are aware, and all Irishmen by facts which most Englishmen try to forget. The student of politics must therefore read history, and particularly the history of those events and habits of thought in the immediate past which are likely to influence the generation in which he will work. But he must constantly be on his guard against the expectation that his reading will give him much power of accurate forecast. Where history shows him that such and such an experiment has succeeded or failed he must always attempt to ascertain how far success or failure was due to facts of the human type, which he may assume to have persisted into his own time, and how far to facts of environment. When he can show that failure was due to the ignoring of some fact of the type and can state definitely what that fact is, he will be able to attach a real meaning to the repeated and unheeded maxims by which the elder members of any generation warn the younger that their ideas are 'against human nature.' But if it is possible that the cause was one of mental environment, that is to say, of habit or tradition, or memory, he should be constantly on his guard against generalisations about national or racial 'character.'