The difficulty of defining pleasure or pain is not the same as the difficulty or impossibility of defining any elementary sensation. For the latter is connected in definite ways with other similar sensations, can be compared and associated with them, and by such association go to make up an object or thing. But pleasure and pain are neither objects nor parts of objects: they cannot be distinguished from or associated with the impressions of the senses so as to constitute an object. They can only be spoken of as an affection of the percipient and active subject, different in kind both from the objects it knows and the acts it performs: each can only be defined as the opposite of the other. Pleasure and pain are not real phenomena with a distinguishable existence of their own, like sensations, conceptions, or actions; they have no trace of objectivity whatever, but are, as Hamilton puts it,[176] "subjectively subjective": "pleasure is not a fact, nor is pain a fact, but one fact is pleasurable, another painful."[177] Pleasure, therefore, is a mere feeling of the subject, concomitant with the sensory or motor presentations which, by reason of their presence to consciousness, we call objects or actions. It is not something by itself which we can choose rather than something else, as we may select a peach instead of an apple. It can only be made the end of conduct in an indirect way. We must aim not at pleasure _per se_, but at objects which we have reason to believe will be accompanied by pleasurable feeling. Pleasure and pain, as it has been urged,[178] are not quantities that can be added and subtracted. It is not the pleasurable or painful feeling, but the perceptional or cognitive elements in the mental state of which it is an element, that admit of plurality and measurement. But we may foresee that one mental state will be accompanied by pleasurable, another by painful feeling, and, on that account, we may choose the former. In a great number of cases we are further able to make a quantitative estimate, and to say that the pleasurable feeling accompanying one object or action is more intense than that accompanying another, and thus to choose one object rather than another, not merely because one is pleasurable while the other is painful, but (in cases where both are pleasurable) because it is supposed that the one will yield more intense or more prolonged pleasure than the other. If this be true, the purely subjective nature of pleasure does not make it impossible for it to be taken as the practical end of conduct for the individual--however inexact and tentative many of its estimates must be--though it will shortly appear that its nature unfits it to be the end on the theory of evolution.