Now that we have likewise reduced philosophies of history to the rank of particular histories, it is scarcely necessary to demonstrate that the demand being made in several quarters for a 'universal' or 'general' history of science is without foundation. For such a history would be impossible to write, even if we were able to identify or compare the history of science with chat of philosophy. But it is doubly impossible both because there are comprised under the name of 'science' such diverse forms as sciences of observation and mathematical sciences, and also because in each of these classes themselves the several disciplines remain separate, owing to the irreducible variety of data and postulates from which they spring. If, as we have pointed out, every particular philosophical problem links and places itself in harmony with all other philosophical problems, every scientific problem tends, on the contrary, to shut itself up in itself, and there is no more destructive tendency in science than that of 'explaining' all the facts by means of a 'single principle,' substituting, that is to say, an unfruitful metaphysic for fruitful science, allowing an empty word to act as a magic wand, and by 'explaining everything' to 'explain' nothing at all. The unity admitted by the history of the sciences is not that which connects one theory with another and one science with another in an imaginary general history of science, but that which connects each science and each theory with the intellectual and social complex of the moment in which it appeared. But even here too we must utter the warning that in thus explaining their true nature we do not wish to contest the right to existence of tables and encyclopædias of the history of science, far less to throw discredit upon the present direction of studies, by means of which, at the call of the history of the sciences, useful Research is stimulated in directions that have been long neglected. Nor do we intend to move any objection to histories of science in the form of tables and encyclopædias on the ground that it is impossible for the same student to be equally competent as to problems of quite different nature, such as are those of the various sciences; for it is inconceivable that a philosopher exists with a capacity equal to the understanding of each and every philosophical problem (indeed, the mind of the best solver of certain problems is usually the more closed to others); or that a critic and historian of poetry and art exists who tastes and enjoys equally all forms of poetry and art, however versatile he be. Each one has his sphere marked out more or less narrowly, and each is universal only by means of his particularity.