A study of the historical development of philosophy both requires and stimulates intellectual detachment. The person who limits himself to the present can easily fall a prey to passing fashions; he becomes a slave of the latest -ism. Intellectually rootless and inexperienced, he succombs to something that may exercise considerable attraction at this particular moment, but that soon withers and passes. For example, Ernst Haeckel's theories once exercised an enormous fascination on all sorts of people; they were even hailed as the definitive word in philosophy. Nowadays they are more likely to cause amusement than anything else. The same may be said of Nietzsche's philosophy, or materialism, or vitalism, or idealism.