The first appeal is to common observation and experience, according to which the four taste qualities,—sweet, sour, salt, and bitter,—stand out as conspicuous classes within which may be placed a great variety of “taste blends.” Thus many substances, while having more or less distinctive flavors, resemble each other in that unanalyzable quality which we call sweetness. The only question here is whether or not we should also include qualities other than these four as ultimate and irreducible. If we refer back to the lists of tastes proposed, we find that many of them are such as can be shown to be analyzable into one or more of these four qualities, plus the intrusion of tactile, thermal, kinæsthetic, and olfactory qualities. Such tastes as nauseating, aqueous, astringent, styptic, putrid, etc., are easily ruled out on this score alone. Introspectively, by simple experimental variation, or by casual observation, the complex character of many of these tastes is easily revealed, and when the non-gustatory components are eliminated the residue falls readily under one or other of our four qualities. These reductions are borne out by definite experiment. The tactile (touch), thermal (temperature), and kinæsthetic (movement) factors are kept constant and reduced to a minimum by applying minute amounts of various solutions to single papillæ or very small regions of the tongue. Smell may be, under these conditions, in great measure eliminated by closing the nostrils with cotton or wax, and by letting the tongue be somewhat advanced beyond its usual position. When these conditions are observed, it is found that the main sense qualities experienced are those of salt, sour, sweet, and bitter, along with such touch sensations as may be unavoidable. Temperature may be eliminated by having the solutions maintained at the temperature to which the tongue is already adapted. The evidence on this point is not absolutely consistent. Some observers, for example, feel impelled to add metallic and alkaline to the group, making six elementary qualities instead of four. Other observers,—most, in fact,—are persuaded that the metallic and alkaline qualities represent mixtures of the salt, sour, sweet, and bitter, along with unavoidable sensations of touch and smell. Thus, by a suitable mixture of strong solutions of salt and sweet substances, the alkaline taste may be very well produced. “It has been suggested that the metallic taste is due to the simultaneous development of salt and sour tastes. The failure to produce exact alkaline and metallic tastes synthetically is in part due to the difficulty of imitating the tactual and other sensations with which they are bound up.” Still other observers are convinced by careful elimination of smell sensation that the unique character of the alkaline and metallic qualities is really a question of odor.