These are psychological facts--not mere theories--which hold true even of the individual experience. But they have led psychologists to the theory, supported by a vast amount of direct experiment, that there is no object or action which can be said to be absolutely and in itself either pleasant or painful.[185] The feeling of pleasure or pain accompanying the object is a function of its intensity in relation to the subject. This proposition cannot, indeed, be fully demonstrated regarding each simple sensation: to the emotions into which intricate relations of perceptions enter, it does not apply, till their complexity has been reduced. Some sensations and perceptions are certainly felt as painful in any intensity in which they are distinctly present to consciousness. But, although this is a real difficulty, it does not seem insuperable. The instances which Mill cites[186] to throw doubt on the generalisation that quality of feeling depends on intensity are unfortunately chosen for his purpose. For--to take his example--the taste of rhubarb is to many not painful but pleasant; and, indeed, every case of acquired taste shows that pleasure and pain can be modified through habit and custom, and suggests that, even in the case of those sensations which are painful in any form we have been able to experience them, there is a degree of intensity below which they would, if experienced, be pleasant. Experiment has proved of the majority even of sensible qualities, and analogy leads us to conclude of all, that there is a degree in which each may be pleasant, and a degree in which each may be painful, and, between them, a--real or imaginary--zero-point of feeling, where there is neither pleasure nor pain. This must, it is true, be received as a hypothesis only; but it is a hypothesis which is suggested by a wide range of facts, and which is able to include even those facts with which it is seemingly inconsistent, by supposing that could their intensity be indefinitely diminished without their passing out of consciousness, these sensations would reach a point after which they would be felt as pleasant and not as painful. Further, experiment shows that this dividing-point which separates the two poles of feeling is not always placed at the same degree of intensity, that it differs not only for every object, but for each individual subject as well, and that it undergoes modification in the course of the subject's development.[187] What is true of sense-perception is still more evident regarding those experiences in which the activity of the subject is more obviously involved. As any function may, if carried beyond a certain degree of intensity, be painful, so any function consistent with life may be a source of pleasure. * * * * * [Sidenote: (_c_) Application of the theory of evolution:] [Sidenote: any conduct consistent with conditions of life will come to be pleasurable;]