The traditional definition of ought is "moral obligation" as defined by multiple online dictionaries. Some authors, like Harris, have defined ought as maximizing expected well being (a fancy way of saying utilitarianism). Other authors, such as Greenspan, define ought as practical reason.
What is an argument that 'ought' could be defined as sentimentalism? Sentimentalism is a moral theory concerning how people know moral truths through their sentiments and emotions.
A few thoughts, probably too quickly. It seems to me that an ought could be understood as the "sense" of the moral sentiment, but in particular it is a moral judgment. The sentiment then correlates with a feeling of shame, responsibility, piety -- perhaps at bottom simply the desire to obey. Moral passions are negative in this sense, relying on guilt and deprivation as implicit motivators. Maybe more simply: to my mind the sense of an ought is the constraint of free action, the separation of bodies and forces from what they can do. In this sense the ought subjugates, interposes, nullifies.
But perhaps the opposite is true as well, at least once we detach the ought from axiomatic moral judgment; maybe this could be explored in terms of Hume's empiricism and the inseparable gulf between an is and an ought. This can also perhaps be read alongside Nietzsche's doubts about ways of life that denounce existence in favor of something else. In other words: there is also an ethical relation of obligation that is independent of moral axioms and the logic of judgment and punishment; a positive order of joy and levity, characterized by an empirical investigation into ways of living, thinking and feeling -- this is ethics in the classical sense as an art of living.
It is the abandonment of moral judgments about reality ("existence is blameworthy") that seems key to me here; an experimental ethics yields a very different spirit of analysis than a transcendental morality that knows everything in advance. Very simply it is less gloomy, less bored -- it lacks the gravity and sad passions that motivate moral responsibilities (bitterness, grief, melancholy, resentment, vengeance and so on.) And so just to complicate this schema a bit, perhaps an absolutely immanent ethics makes use of a certain "transcendent" moment in its own way. But there is a different relation; it is no longer a starting point (Spinoza makes this very clear -- it is important not to start out with the idea of God, but rather to reach it as swiftly as possible...)
To the degree that aesthetics works largely thorough sentiment, I would agree with this, following Nietsche's notion that the proper basis of consistent decision making is not rule-following morality but an aesthetics of politics. One should strive "to make of one's self a work of art".
I think the idea behind 'ought' is in its form, in two different ways. It is a subjunctive form of 'owe'. The notion of 'subjunction' and he verb 'to owe' capture separately the two concepts of morality at the base of the culture that gave us the word: fealty and propitiation.
The words roots are Germanic and Germanic law comes from codes where money or other hard goods were traded to bypass obligations of revenge, as in, for instance, the Salic code of weregelt. This is in the context of a warrior nobility culture where the military leadership who defended the land owned it, and those who worked the land paid them rent for their protection. Our words 'forgiveness', and 'duty' are forms of 'given for' meaning 'paid off' and 'due', as in 'rent due'.
Nietzsche seems 'mean' to many people for being this reductive. But we need to take the more basic forms of our real politics seriously. We resist reducing our systems of obligation to tribal economics, but that is what they are. We are obligated to compensate our culture for the cost of producing us, and the given culture states what form that compensation takes. Generally it requires that we not make life too hard for others in the culture, and not allow the culture to be needlessly endangered.
Beyond simply tracing history, and considering the pragmatics, it is a bad idea to give global definitions to words like 'ought', and to realize that, as cultural products, they are completely contextual. What captures a context best is an aesthetic sense inculcated by immersion in human interactions in the culture. And in the end, that is all we have to base our sense of 'duty' upon.
To the degree there is biologically shared human culture, we can make some basic assumptions about what 'ought to' trigger our basic judgmental emotions, but to reduce all culture to that most basic level is basically destructive.
