Whoever devotes himself to the study of so comprehensive a science must try never to lose sight of two virtues: conscientiousness and modesty. The older we grow, the more we feel the limits of human knowledge. “Good care is taken,” as Goethe said, “that trees should not grow into the sky.” Every one of us can make himself real master of a small field of knowledge only, and what we gain in extent, we inevitably lose in depth. It was impossible that Bopp should know Sanskrit like Colebrooke, Zend like Burnouf, Greek like Hermann, Latin like Lachmann, German like Grimm, Slavonic like Miklosich, Celtic like Zeuss. That drawback lies in the nature of all comparative studies. But it follows by no means that, as the French proverb says, _qui trop embrasse, mal étreint_. Bopp’s “Comparative Grammar” will always mark an epoch in linguistic studies, and no one has accused the old master of superficiality. There are, in fact, two kinds of knowledge; the one which we take in as real nourishment, which we convert _in succum et sanguinem_, which is always present, which we can never lose; the other which, if I may say so, we put into our pockets, in order to find it there whenever it is wanted. For comparative studies the second kind of knowledge is as important as the first, but in order to use it properly, the greatest conscientiousness is required. Not only ought we, whenever we have to use it, to go back to the original sources, to accept nothing on trust, to quote nothing at second-hand, and to verify every single point before we rely on it for comparative purposes, but, even after we have done everything to guard against error, we ought to proceed with the greatest caution and modesty. I consider, for instance, that an accurate knowledge of Sanskrit is a _conditio sine quâ non_ in the study of Comparative Philology. According to my conviction, though I know it is not shared by others, Sanskrit must forever remain the central point of our studies. But it is clearly impossible for us, while engaged in a scholarlike study of Sanskrit, to follow at the same time the gigantic strides of Latin, Greek, German, Slavonic, and Celtic philology. Here we must learn to be satisfied with what is possible, and apply for advice whenever we want it, to those who are masters in these different departments of philology. Much has of late been said of the antagonism between comparative and classical philology. To me it seems that these two depend so much on each other for help and advice that their representatives ought to be united by the closest ties of fellowship. We must work on side by side, and accept counsel as readily as we give it. Without the help of Comparative Philology, for instance, Greek scholars would never have arrived at a correct understanding of the Digamma--nay, a freer intercourse with his colleague, Bopp, would have preserved Bekker from several mistakes in his restoration of the Digamma in Homer.