There are two kinds of organic response--one more or less permanent, the other temporary and transient. We have already seen something of the former, by which the tissues (the epidermis of the oarsman's hand, and the muscles of his arm) respond to the call made upon them. The response is here gradual, and the effects on the organism more or less enduring. This, however, is not the kind of response with which we have now to deal. What we have now to consider is that rapid response, transient, but of the utmost importance, by means of which the organism directly answers to certain changes in the environment by the performance of certain activities. The parts specially set aside and adapted to receive special modes of influence of the environment are the sense-organs. We human folk get so much pleasure from and through the employment of our sense-organs, that it is important to remember that the primary object of the process of reception of the influences from without was not the æsthetic one of ministering to the enjoyment of life by the recipient organism, but the essentially practical one of enabling that organism to respond to these influences. In other words, the _raison d'être_ of the sense-organs is to set agoing suitable activities--activities in due response to the special stimuli. In this chapter we shall consider the modes in which the special sense-organs are fitted to receive the influences of the environment, deferring to a future chapter the consideration of the resulting activities. For the present we take these activities for granted, observing them only in so far as they give us a clue to the sense-reaction by which they are originated. In this chapter, too, we shall deal, for the most part, with the physiological aspects of sensation. In all other organisms than ourselves, that is to say, than each one of us individually for himself, the psychological accompaniments of the physiological reactions of the sense-organs are matters of inference. Still, so closely and intimately associated are the physiological and the psychological aspects, that the exclusion of all reference to the latter would be impracticable, or, if practicable, unadvisable. What is practicable and advisable is to remember that, even if the two are mentioned in a breath, the physiological and the psychological belong to distinct orders of being. In addition to the time-honoured "five senses," there are certain _organic sensations_, so called, which take their origin within the body. These are, for the most part, somewhat vague and indefinite. They do not arise immediately and in direct response to changes in the environment, but indicate conditions of the internal organs. Such are hunger, thirst, nausea, fatigue, and various forms of discomfort. Although they are of vital importance to the organism, prompting it to perform certain actions or to desist from others, they need not detain us here.