The fifth factor that might account for individual differences is environment. By environment we mean any influence brought to bear on the individual. The same difficulty has been met in attempting to measure the effect of environment that was met in trying to measure the effect of inner nature--namely, that of testing one without interference from the other. The attempts to measure accurately the effect of any one element in the environment have not been successful. No adequate way of avoiding the complications involved by different natures has been found. One of the greatest errors in the method of working with this problem has been found just here. It has been customary when the effect of a certain element in the environment is to be ascertained to investigate people who have been subject to that training or who are in the process of training, thus ignoring the selective influence of the factor itself in original nature. For instance, to study the value of high school training we compare those in training with those who have never had any; if the question is the value of manual training or Latin, again the comparison is made between those who have had it and those who haven't. To find out the influence of squalor and misery, people living in the slums are compared with those from a better district. In each case the fact is ignored that the original natures of the two groups examined are different before the influence of the element in question was brought to bear. Why do some children go to high school and others not? Why do some choose classical courses and some manual training courses? Why are some people found in the slums for generations? The answer in each case is the same--the original natures are different. It isn't the slums make the people nearly so often as it is the people make the slums. It isn't training in Latin that makes the more capable man, but the more intellectual students, because of tradition and possibly enjoyment of language study, choose the Latin. It is unfair to measure a factor in the environment and give it credit or discredit for results, when those results are also due to original nature as well, which has not been allowed for. It must be recognized by all those working in this field that, after all, man to some extent selects his own environment. In the second place, it must be remembered that the environment will influence folks differently according as their natures are different. There can be no doubt that environment is accountable for some individual differences, but just which ones and to what extent are questions to which at present the answers are unsatisfactory.