This truth is no doubt hard to see in the case of purely technical science; in books of wider range, such as Darwin's for instance, it is easy for any reader to feel the presence of a really great mind, producing inspiration of a different sort from that of the most excellent up-to-date examination text-book. In philosophy, religion, poetry, and the highest kinds of art, the greatness of the author's mind seems as a rule to be all that matters; one almost ignores the date at which he worked. This is because in technical sciences the element of mere fact, or mere knowledge, is so enormous, the elements of imagination, character, and the like so very small. Hence, books on science, in a progressive age, very quickly become 'out of date', and each new edition usually supersedes the last. It is the rarest thing for a work of science to survive as a text-book more than ten years or so. Newton's _Principia_ is almost an isolated instance among modern writings. Yet there are some few such books. Up till about the year 1900 the elements of geometry were regularly taught, throughout Europe, in a text-book written by a Greek called Eucleides in the fourth or third century B. C.[1] That text-book lasted over two thousand years. Now, of course, people have discovered a number of faults in Euclid, but it has taken them all that time to do it. [1] Since this paper was first written _Euclid_, Book I, in the Greek, has been edited with a commentary by Sir Thomas Heath (Cambridge Press, 1920). It is full of interest and instruction. Again, I knew an old gentleman who told me that, at a good English school in the early nineteenth century, he had been taught the principles of grammar out of a writer called Dionysius Thrax, or Denis of Thrace. Denis was a Greek of the first century B. C., who made or carried out the remarkable discovery that there was such a thing as a science of grammar, i. e. that men in their daily speech were unconsciously obeying an extraordinarily subtle and intricate body of laws, which were capable of being studied and reduced to order. Denis did not make the whole discovery himself; he was led to it by his master Aristarchus and others. And his book had been re-edited several times in the nineteen-hundred odd years before this old gentleman was taught it.