if his word-formations had been _à priori_. Man is, in fact, a psychological as well as a logical being. If there is to be any practical outcome, we must, therefore, under all circumstances base our work on the psychological principle of internationality. It is only this which confers on the auxiliary language the quality of being easily learnt and spoken, which is unconditionally necessary for its practical use in science, as in other departments of life. Such systems are called _à posteriori_, and experience shows that the more _à posteriori_ elements are contained in an international language the more it conforms to Jespersen's fundamental principle of _the greatest ease for the greatest number of people_. But, one may argue, does it not follow from this that the best solution would be the introduction of a _national_ language into science? Certainly not, for this would not offer the greatest facility to the greatest number of people, because the formation of the so-called idioms, which, apart from grammatical difficulties, hinder the learning and use of a language, would in the case of many national languages interfere with the internationality of the vocabulary. These idioms have a very similar effect to the _à priori_ word formations, and diminish the intelligibility, lucidity, and facility of logical expression. The only international auxiliary language which will be of practical use in science will be constructed according to the _à posteriori_ principle of maximum internationality, and will be almost or entirely free from idioms. If we add to this that it must possess that logical clearness of expression which we have described above as the second criterion, we have the general conditions which must be satisfied by an international language suitable for science. Apart from the practical value of the principle of internationality, there exists in science another very special reason for regarding it as a necessary condition to be satisfied by an international auxiliary language. We may inquire, in fact, from a purely scientific standpoint, how far the systems which have been devised up to the present have adjusted themselves to the international language which already exists in science. For all the thousands of words in scientific and technical nomenclature which, apart from their nationality, the scientific men of all countries have been inventing for centuries according to very uniform principles, as well as the likewise largely international expressions of "unofficial" nomenclature, form a possession of modern scientific civilisation of such magnitude, importance, and value, that it cannot on any account be sacrificed. On the contrary, all these words, as well as many similar ones derived from daily life, form the true, natural, and practical basis of international language.