“We seldom ask, ‘Why should I believe what I see to be true?’ but we frequently ask, ‘Why should I do what I see to be right?’
“It is easy to reply that the question is futile, since it could only be answered by a reference to some other recognized principle of right conduct, and the question might just as well be asked as regards that again, and so on. But still we do ask the question widely and continually, and therefore this demonstration of its futility is not completely satisfactory: we require besides some explanation of its persistency.
“One explanation that may be offered is that, since we are moved to action not by moral judgment alone, but also by desires and inclinations that operate independently of moral judgment, the answer which we really want to the question ‘Why should I do it?’ is one which does not merely prove a certain action to be right, but also stirs in us a predominant inclination to do the action.
“That this explanation is true for some minds in some moods I would not deny. Still I think that when a man seriously asks ‘why he should do’ anything, he commonly assumes in himself a determination to pursue whatever conduct may be shown by argument to be reasonable, even though it be very different from that to which his non-rational inclinations my prompt. And we are generally agreed that reasonable conduct in any case has to be determined on principles, in applying which the agent’s inclination–as it exists apart from such determination–is only one element among several that have to be considered, and commonly not the most important element. But when we ask what these principles are, the diversity of answers which we find manifestly declared in the systems and fundamental formulae of professed moralists seems to be really present in the common practical reasoning of men generally; with this difference, that whereas the philosopher seeks unity of principle, and consistency of method at the risk of paradox, the unphilosophic man is apt to hold different principles at once, and to apply different methods in more or less confused combination. If this be so, we can offer another explanation of the persistent unsatisfied demand for an ultimate reason, above noticed. For if there are different views of the ultimate reasonableness of conduct, implicit in the thought of ordinary men, though not brought into clear relation to each other,–it is easy to see that any single answer to the question ‘why’ will not be completely satisfactory, as it will be given only from one of these points of view, and will always leave room to ask the question from some other.” — Henry Sidgwick, The Methods of Ethics, p.6