I think the Golden Rule and its variations "do unto others, etc" covers a lot of moral ground for both theists and non theists alike. Or maybe the United Nations Declaration of Human Rights will do.
I am not sure notions of eternal punishment or reward are that admirable.

Morality can be grounded in rational autonomy/free will (cf. Kant's Groundwork): autonomy is the condition for human reason, and the moral law is delineated in human reason. This is not constructivism, since moral facts are held to be discovered in reason, and not constructed by it. And this is not "ultimately subjective," since the faculty of reason is present among all human beings.

Well, Buddhists are 'non-theists'. Buddhist morality is grounded in the original Buddhist teachings, and also the subsequent elaboration and development of the principles in those teachings by generations of Buddhists. As it has developed quite separately to the Christian tradition, it views ethics and ethical principles in a very different way to the Christian, although I think it is fair to say that Buddhist and Christian ethics are quite convergent in many important respects.

But ultimately Buddhist ethics are grounded in the reality of karma - that all intentional actions have consequences - and founded on the inter-dependent principles of Śīla (morality), Prajñā (wisdom) and Samadhi (meditative absorption). These are compared to the 'three legs of the tripod', three legs being necessary for the structure to stand. However the ultimate aim of Buddhist teaching is, in my opinion, transcendent.

The spiritual values advocated by Buddhism are directed, not towards a new life in some higher world, but towards a state utterly transcending the world, namely, Nibbana. In making this statement, however, we must point out that Buddhist spiritual values do not draw an absolute separation between the beyond and the here and now. They have firm roots in the world itself for they aim at the highest realization in this present existence. Along with such spiritual aspirations, Buddhism encourages earnest endeavor to make this world a better place to live in.

Buddhism and the God Idea, Nyanaponika Thera.

Grounds
To assume an ontological answer is to assume the reality of socially constructed norms.
To be a moral realist then is to accept the reality of socially constructed norms as objectivly reality

Under socially constructed norms, then slavery would have been moral in its time? Essentially morality or ethics becomes absolute moral relativism, a result that many would reject.

Intuition. Moral naturalism is a lost cause, and divine command theory is either ad hoc or fails from Euthyphro.

Under socially constructed norms, then slavery would have been moral in its time?

No, I don't think anyone can go back in time, literally or figuratively to say what reality was for the individually at various times throughout history. We must judge history based on our own normative understandings and valuations. Similarly we now judge Newton's theories as an incomplete understanding of the world based on our point if view, not his.

Essentially morality or ethics becomes absolute moral relativism, a result that many would reject.

Many would accept a $5.00 bill as payment for a $5.00 debt. Offering a debtor less than what is owned for no good reason might invoke their laughter. Not all norms are all that normative, some norms have evolved along with our understandings, and these continue play a crucial part in the social construction of reality.

The question of this topic has got me wondering about theists - surely they are in no different a position to non theists when it comes to "grounds for morality" ... except for theists who admit to blindly following their religion's stated morality simply because it is in "the book" or whatever.

rational autonomy — Brian A

(Y)

We generally like freedom and dislike harm (including other animals), and that can, and do, inform judging actions in terms of morality.
Which hardly are matters of arbitrary, ad hoc opinion, not mere whims of the moment; who ever called liking freedom or disliking harm random or discretionary anyway?
If you require myths and commands to understand that, then there's a good chance you're a bit scary. :)
The objective versus subjective thing is misleading from the get-go.

This is an ontological, not an epistemological question about ethics. I am aware atheists can be very moral beings. — Modern Conviviality

When you are asking for the grounds of a position, i.e. "Why do you hold to that position?" you are, by definition, asking an epistemological question. To insist that it is an ontological question is to beg the question. You are smuggling some kind of an answer into your question: the only "grounds" you will accept must be some kind of "thing" or fact in the world, right? I suppose this leads to your next stipulation:

This is a question for non-theists who hold to objectivity in ethics (moral realists) - e.g. it is always true that murdering someone for no reason is morally wrong, etc. — Modern Conviviality

Moral realism is usually understood as the statement that (a) moral claims are statements of facts (more than just facts about our own thoughts and feelings), of the way the world is (outside our heads), and (b) at least some moral claims are true. Is this what you mean by moral realism?

Grounding morality in: evolution (naturalistic fallacy), sentiment (subjectivity), or human reason (ultimately subjective, for whose reason are we speaking of? And human reason, limited as it is, cannot construct moral laws) - seems incoherent. — Modern Conviviality

Why do you think so? "Incoherent" means, strictly speaking, contradictory. What contradictions do you see in these positions?

ETA: Some of the answers posted here (, , ) propose to ground all or most of morality in some particular moral dictum (the Golden Rule, the primacy of personal freedom), but these are not really answers to the question posed in the OP. These are proposals for theories of morality that reduce most moral claims to some fundamental moral principle. But these proposed grounds are themselves moral principles, and so they cannot ground all of morality.

They ground morality in their own sinful desires.

Suffering and happiness; barring, of course, exceptions like sadism and masochism. To think of it, even religion resorts to the suffering-happiness paradigm in formulating moral theory. A religion that makes suffering good and happiness bad simply doesn't exist, proving my point.

Answering for myself (I am not a theist, but I am not sure that I am a "moral realist," because this notion is not very clear to me), I don't seek to ground my moral convictions in anything. I don't think that, as far as moral claims go, there is anything more fundamental than moral convictions.

Some of my moral judgments are more secure than others, and at times I seek to ground some less secure opinions in more secure, more fundamental convictions. But, as I wrote above, this kind of query cannot provide the grounds for morality as a whole.

We might try to explain morality as a natural - or a supernatural - phenomenon, but this can only tell us what is, not what ought to be.

"Does a man bathe quickly (early)? do not say that he bathes badly, but that
he bathes quickly. Does a man drink much wine? do not say that he does this bad-
ly, but say that he drinks much. For before you shall have determined the opinion,
how do you know whether he is acting wrong? Thus it will not happen to you to
comprehend some appearances which are capable of being comprehended, but to assent to others" - Epiktetos

The understanding that to desire or disturb myself with what is beyond my control serves only to make me subject to others, and unhappy.

Yes, but how can the golden rule, or any moral rule or law, on a non-theistic worldview, be more than an illusion or construct

Interesting. I agree that intuition is how we directly apprehend the moral realm and a SENSE of moral objectivity. Viz. we feel the phenomenological weight of an inescapable moral reality. But I think intuition by itself does not constitute an argument for the reality of an objective moral realm. Intuition gives us a pragmatic case for the rationality of moral objectivity.

On an aside, I think DCT is the closest thing we have to an ontological grounding of morality

Very interesting. But how are 'moral facts held to be discovered in reason'? Again, its not that reason can apprehend moral truths, we both agree on that. Its that moral laws are somehow part of the rational/epistemic enterprise itself - viz. a moral law is just a true proposition, say. Am I getting this right?

But I think intuition by itself does not constitute an argument for the reality of an objective moral realm. — Modern Conviviality

Might want to look into Simon Blackburn's quasi-realism. It still runs into the difficulty of justifying how exactly morality is objective, but it manages to seem to fuse projectionist and non-cognivitism into a distinctly cognitivist morality. Our attitudes towards things are projected onto the world and in turn we "perceive" this very projection and formulate truth-apt statements.

But in general with a lot of philosophical debates and meta-ethical ones you get four camps, some form of non-naturalism, naturalist reductionism, expressivism and eliminativism, the former two being realist and the latter two being anti-realist. It's not perfectly cut like this in real life but in general it's a basic template that most issues end up being structured as.

Buddhist morality is grounded in the original Buddhist teachings — Wayfarer

Again what your talking about is moral epistemology, not moral ontology. Many moral teachers may teach many different moral doctrines. This does not in any way broach the question of the ground of moral truths.

Says you. No argument here, only bald assertion.

The fundamental question of Buddhism is: what is the source and the end of suffering. Nirvāṇa, as 'the end of all suffering', is exactly that: the end of all suffering. It is, therefore, a supreme good, and one around which the Buddhist teachings serve.

Says you. No argument here, only bald assertion. — Wayfarer

But I'm not trying to forward any argument, just explicating and making important distinctions.

This does not in any way broach the question of the ground of moral truths. — Modern Conviviality

That is an assertion, or, if you like, a proposition, by which you claim to have essentially eliminated Buddhism from answers to your original question.

But ultimately Buddhist ethics are grounded in the reality of karma — Wayfarer

This is very new for me. I'm not going to attempt to respond critically, only to ask how does the Buddhist know that 'all intentional actions have consequences'?

how does the Buddhist know that 'all intentional actions have consequences'? — Modern Conviviality

I would think through a combination of observation and inference. The word 'karma' comes from the root word for 'hand' (kr-) - the implication being 'action' or 'deed', and its consequences. It is not hard to observe this relationship in day to day life.

To turn the question around - how can the link between action and consequence NOT be fundamental to a moral philosophy? The idea that actions don't have moral consequences is surely one of the main grounds that theistic philosophies fault atheism for, isn't it?

And besides, there are biblical verses that can be cited as being an implicit recognition of the same principle, specifically, 'as you sow, so shall you reap', which is practically folk wisdom (although no less true for that.)

The deeper issue for Buddhism is, if karma is always to have consequences, then it must, somehow, extend beyond physical death; because if physical death 'wipes the slate clean', so to speak, then there are no consequences for evil actions beyond what occurs in this physical existence. That, of course, is a deep question; suffice to say that in theistic traditions, it is dealt with in relation to doctrines of eschatology, i.e. 'the fate of the soul after death', typically depicted in terms of heaven and hell (and limbo, in traditional Catholicism).

I don't want to take the thread in that direction, beyond observing that both theistic and Buddhist religious cultures acknowledge the concept of 'a life beyond' as essential to their ethical doctrines.

trying to rationally justify divine command while rejecting the authority of reason is incoherent. — Πετροκότσυφας

I appreciate this criticism. But I'm not rejecting the authority of reason, reason is all we have to fall back on as philosophers. I'm making the point that placing the building of moral objectivity on the foundations of human reason is constructivist, because surely the moral realm existed before humans did. If morality came into existence with man, how can it be objectively binding (i.e. law-like).

if morality came into existence with man, how can it be objectively binding (i.e. law-like). — Modern Conviviality

Excellent question. I don't think anyone else in this thread has an answer to it.

Since morals are the rules to live by in the culture you find yourself in, my morality comes from recognizing that like to be free and not in prison, so I follow the rules of the culture I find myself in.

As a member of a social species, I need others around me to be happy and recognize that to piss them off would be to possibly lose them and make me unhappy.

First off, I would want to say that I do not believe in any sort of "objective" morality. Or to put it another way, I don't think that there any absolute fact as to what is right or wrong. However, that doesn't mean that I don't think that there doesn't exist any morality at all. Clearly we do have a deep-rooted sense as to what is right or wrong, and I believe that this is grounded in our own capacity as human beings to empathize and feel compassion for others. We are social beings after all, so it's not surprising that we have evolved with an inborn conscience. This, I think, is where laws such as the golden rule come from, and why they are so pervasive across different cultures.

Short of Platonism, are these all the options a non-theist has at his disposal? — Modern Conviviality

The approaches you don't appear to be exploring are (a) virtue ethics, a process of learning good action grounded in the interplay between your reason and experiences with the social practices you find around you; and (b) a grounding of ethics in how we are with and for each other, the I-you relation, which many Continental philosophers are into, but which has been propelled into the analytic way of doing ethics by Stephen Darwall's 'The Second Person Standpoint'.