Convention is a very real and wellnigh irresistible power. Not a few of our most cherished valuations--artistic, religious, political and social--are conventional fetishes which have been slowly evolved in the course of a great number of years as the result of determining factors, for the most part accidental and forgotten, and probably called into existence for totally different and unconnected reasons. Yet the appropriate emotional reaction, evoked by the association of an object with such a conventional valuation or sentiment, may be just as keenly and genuinely felt as though it resulted from the awakening of some instinctive or innate law of our nature. Impressionability is not a quality to be despised, but on the contrary to be carefully guarded from contamination. It is by means of emotion that all pleasure and pain, all aversion and attraction, and all sense of the æsthetic is recorded by the senses. Emotional sensibility may be compared to an instrument that may be so finely made that it is capable of registering the most delicate and exact vibrations so that any harsh sound will injure it, while, on the other hand, it may be made of a texture so coarse that it will respond instantly and indiscriminately to any loud and crude noise. This instrument has an inherent quality of excellence with a potentiality of exactness that may be developed in a great variety of directions. The many factors which play a part in æsthetic appreciation have been abundantly explored by psychological writers.[76] They have traced the great variety of ways in which art can be the means of evoking sympathetic emotions by connecting its subject with the inexhaustible interest in personality. They have cited the part played in inducing pleasurable sensations in music by the association of range, depth of tone and pitch with the expression of human passions; and in pictorial art, the appeal to muscular sensibility by suggested associations with movement and form, or the effect of straight lines and rounded forms in inducing sensations of vigour and repose. More obvious are the appeals to the sexual instincts. There are also associations that give beauty to colours, pleasurableness to those tints that suggest youth, health, vigour and feminine charm. It is easy to understand the agreeableness of symbols of strength and solidity; the restfulness of economy in presentation, the pleasing effect of contrast and symmetry, variety and unity, of balance and the laws of proportion and musical ratios, or of harmony and regularity. The laws of relativity or comparison and of familiarity and strangeness are factors which play a part in all appreciation. Finally, there is a more exclusively intellectual pleasure in the process of analytical valuation of artistic production.