§6. These considerations make clearer the extent of the assumptions of Empirical Quantitative Hedonism, stated in the preceding chapter: viz. (1) that our pleasures and pains have each a definite degree, and (2) that this degree is empirically cognisable. Firstly, if pleasure only exists as it is felt, the belief that every pleasure and pain has a definite intensive quantity or degree must remain an a priori assumption, incapable of positive empirical verification. For the pleasure can only have the degree as compared with other feelings, of the same or some different kind; but, generally speaking, since this comparison can only be made in imagination, it can only yield the hypothetical result that if certain feelings could be felt together, precisely as they have been felt separately, one would be found more desirable than the other in some definite ratio. If, then, we are asked what ground we have for regarding this imaginary result as a valid representation of reality, we cannot say more than that the belief in its general validity is irresistibly suggested in reflection on experience, and remains at any rate uncontradicted by experience.
But secondly, granting that each of our pleasures and pains has really a definite degree of pleasantness or painfulness, the question still remains whether we have any means of accurately measuring these degrees. Is there any reason to suppose that the mind is ever in such a state as to be a perfectly neutral and colourless medium for imagining all kinds of pleasures? Experience certainly shows us the frequent occurrence of moods in which we have an apparent bias for or against a particular kind of feeling Is it not probable that there is always some bias of this kind? that we are always more in tune for some pleasures, more sensitive to some pains, than we are to others? It must, I think, be admitted that the exact cognition of the place of each kind of feeling in a scale of desirability, measured positively and negatively from a zero of perfect indifference, is at best an ideal to which we can never tell how closely we approximate. Still in the variations of our judgment and the disappointment of our expectations we have experience of errors of which we can trace the causes and allow for them, at least roughly; correcting in thought the defects of imagination. And since what we require for practical guidance is to estimate not individual past experiences, but the value of a kind of pleasure or pain, as obtained under certain circumstances or conditions; we can to some extent diminish the chance of error in this estimate by making a number of observations and imaginative comparisons, at different times and in different moods. In so far as these agree we may legitimately feel an increased confidence in the result: and in so far as they differ, we can at least reduce our possible error by striking an average of the different estimates. It will be evident, however, that such a method as this cannot be expected to yield more than a rough approximation to the supposed truth.