We are able to picture to ourselves the difficulties which assailed man in his efforts to express his impressions in primitive times, since we find ourselves at times struggling with the same difficulties, and there are occasions when we struggle in vain, we do not conquer the difficulty. Sensations which are subjective and personal are of all others the most difficult to define, since we lack words to express what is from its nature purely personal; and yet we have frequently occasion to mention them, how can we best express ourselves? As the required word does not seem forthcoming we have recourse to metaphor, and almost unconsciously we use terms borrowed from external phenomena connected with the sense of hearing, of smelling, and of tasting, and which for the most part are acts or conditions in the domain of the sense of sight. Our old acquaintances the roots, whose meanings are to cut, to pinch, to bite, to burn, to hit, to sting, to soften, having formed the base of the adjectives sharp, sweet, keen, burning, we use these to describe certain sensations. We do not know how better to particularise a physical pain than by comparing it to something that tears, cuts or stings. But if certain physical ills, certain colour perceptions, certain impressions of sharpness, sweetness and heat experienced when tasting various foods find metaphorical expression in external acts, there still remains a whole category of simple ideas for which no words can be found. There are certain sensations of taste which cannot be expressed in words. Yesterday I ate a pear, to-day I have eaten a peach; I am quite capable of distinguishing the special flavour of each, but finding nothing in the world of facts with which to compare them, I am without words to apply to them, and it would be as impossible for me to convey an idea of the flavour to any one who had never eaten a pear or a peach as to make any person understand if I spoke in a language which was unknown to him. Since all words that succeed in expressing our sensations are drawn from external phenomena, we are in a position to know the origin and historic past of these words. But I cannot thus easily foresee even the near future of some of these words. The sound of the clarionet and that of the hautbois, the whistling of the wind, the whisper of the waves, the yellow of the straw and that of the lemon, the green of the emerald and the blue of the sky, all characterise objects belonging to the material world; but if these words: clarionet and hautbois, wind and waves, straw and lemon, emerald and sky, which alone enable us to define clearly to our minds certain sounds and certain colours were lacking in our vocabulary, I do not know how a musician could have composed a symphony, or an artist painted his picture, although the creation of both works of art proceeds equally from personal inspiration invisible to the eye.