The intuition of moral obligation
H.A. Prichard's ethical theory is a form of intuitionism. You can get a first sense of his position from the following commentary :
Prichard's positive doctrine, as already expressed in the 1912 article,
was that we have immediate knowledge of particular moral obligations in
particular circumstances, and that such knowledge no more requires justification or derivation than any other kind of immediate knowledge. Just as the
theory of knowledge can deal with scepticism only by inducing people to
notice instances of genuine knowledge, so the principal task of moral philosophy is to recall people from unnecessary moral doubt or sophistication
to the immediacy of the apprehension of obligation. (D. J. B. Hawkins, 'The Ethics of H. A. Prichard', The Philosophical Quarterly (1950-), Vol. 1, No. 3 (Apr., 1951), pp. 242-247 : 243.)
Prichard's central view is that we simply apprehend a certain action as obligatory. He does not explain the moral epistemology that supports this claim, i.e. how we intuit a moral obligation. But in fairness to him, I think it does follow that if I simply apprehend a moral obligation - say, to give money to this or that homeless person - then inclination or self-interest or the deduction of the obligation from a general moral principle containing an 'ought' from which I derive the conclusion that I ought (as a moral obligation) to give money to this or that homeless person, play no role in the apprehension of moral obligation.
The intrinsic goodness of the action can fulfil no role because it provides a motive for doing the action. But an action done under moral obligation is done, not from a motive, but from the mere apprehension that it is obligatory.
Motive, moral worth and moral obligation
Prichard accepts that the motive of an action determines its moral worth; but insists that moral worth has nothing to do with moral obligation.
A further extract might help. It isn't entirely easy reading but I'll comment at the end :
According to Prichard and Ross, obligation is independent of moral
worth both in the sense that the rightness of an action can not be analyzed in terms of moral worth and in the sense that one can not infer
the rightness of an action from its displaying moral worth or vice versa.
The main reason for this independence is that the moral worth of an
action depends on the motives from which it is done, whereas the
rightness of an action does not. As a result, it is possible to act from a
motive that gives an action moral worth without having done something that is right, and it is possible to do something right without
having done it from a motive that would give the action moral worth.
That the moral worth of an action depends on the motive from which
it is done also provides the basis for intuitionists' arguments that rightness can not be analyzed in terms of moral worth. It has seemed to
follow from this dependence that any analysis of rightness in terms of
moral worth commits one to the view that what one ought to do is to
act from a certain motive, one that would give one's action moral
worth. Intuitionists have offered a number of reasons for thinking that
this latter view is untenable. First, if the motive in question is the
motive of duty, then the proposed analysis will be circular. Second,
any such analysis leads to an infinite regress. If what is right is acting
from a good motive, and if acting from a motive is itself an action, then
it will be right to act from a good motive from a good motive, and so on
ad infinitum. Third, any such analysis commits one to the possibility of
being moved to be moved (or of willing to will), and that is unintelligible. Finally, any such analysis violates the principle that 'ought'
implies 'can'. In order to act from a motive, I must have it; but motives
are not the sorts of things that I can call up at will. From all of this it
has seemed to follow that whatever it is that makes an action obligatory
or right, it is independent of whatever would allow an action to display
moral worth. (Norman O. Dahl, 'Obligation and Moral Worth: Reflections on Prichard and Kant', Philosophical Studies: An International Journal for Philosophy in the Analytic
Tradition, Vol. 50, No. 3, Symposium on Rationality and Moral Values (Nov., 1986), pp. 369-
399 : 369-70.)
I think a fair gloss on this is to say that in Prichard's view certain actions just are morally obligatory, a matter of duty. He believes that we can, but 'how' he doesn't say, apprehend that an action is obligatory. Obligation is unconditional on anything - save the apprehension that it is a matter of duty. It could not be conditional on motivation because motivation is contingent. I do not have full control over my motives. So if moral obligation depended on motivation it could not be unconditional in the right way; it would be dependent on the chance or fortuity of my motivation.
Prichard is fully prepared to concede moral worth to actions done from certain kinds of motivation. If I give money to the homeless from compassion, this action has moral worth. Moral worth is important but it is quite distinct from moral obligation, which is a requirement apprehended as binding whatever one might be motivated to do.