E.G.

Consider:

The Darwall-Dancy definition of moral reasons: A reason to φ is a moral reason to φ =def it is the sort of reason that can, in the right circumstances, give rise to a moral obligation to φ.

I suspect this definition fits with common talk. For example, I think regular people will say that (a) the fact that you can save lives by donating to the Against Malaria Foundation supplies a moral reason to donate. And I think regular people will also say that (b) the fact that you can save lives by donating to the Against Malaria Foundation can, in the right circumstances, make it the case that you are morally obligated to donate. And I think it's at least not crazy to suggest that, when (a) is said, it's just shorthand for (b).

But my main reason to like the Darwall-Dancy definition is not that it fits with common talk. I am not sure that 'moral reason' has a determinate meaning in common talk. It may be that 'moral reason' is mainly just a piece of philosophers' jargon.

My main reason to like the Darwall-Dancy definition is that I think it is useful. It seems to me that there are certain sorts of reasons that do in fact have the power to give rise to moral obligations, and I think it is handy to have a term for such reasons.

***

In The Moral Universe, BC&S-L offer a different definition of 'moral reason.' They say:

A reason R to φ is a reason of kind K =def [R favors φ] is fully grounded in a fact of kind K, together with R.

And this means that, according to them,

The TMU definition of 'moral reason': A reason R to φ is a moral reason =def [R favors φ] is fully grounded in a moral fact, together with R.

They illustrate the TMU definition of 'moral reason' with the following example:

[C]onsider the fact [The bottle contains poison]. According to the schema, this is a moral reason to keep the bottle from a child just in case its correlative reason-fact,

[That the bottle contains poison favors keeping it from a child],

is fully grounded in the fact that the bottle contains poison together with a moral fact such as

[One is morally required to keep poison from a child].

Suppose the correlative reason-­fact holds, and is fully grounded in the reason in combination with this moral fact. It would follow that [The bottle contains poison] is a moral reason to avoid giving the bottle to a child.

I do not like the TMU definition of 'moral reason.'

What I want to say is that [That the bottle contains poison favors keeping it from a child] is at least partially, and perhaps fully, grounded in facts about children's interests. Poison-containing bottles are dangerous to children. This is the main thing that makes it the case that, if the bottle contains poison, this favors keeping the bottle away from a child. And I am inclined to deny that [That the bottle contains poison favors keeping it from a child] is even partially grounded in any moral facts such as [One is morally required to keep poison from a child].

The TMU definition of 'moral reason' says that, once I've said these things, then I should say that [The bottle contains poison] is not a moral reason to keep the bottle from a child. That seems like a bad commitment to me.

We can, if we want, use 'moral reason' in the way BC&S-L propose but I think that if we do this then we end up with a piece of jargon that is not very useful and may not even refer to anything at all, as I have some doubt that favoring relations are ever grounded in moral facts.

***

I think it is interesting that both the Dancy-Darwall definition of 'moral reason' and the TMU definition of 'moral reason' seem to come along with heavy-duty presuppositions about how different sorts of moral and normative phenomena are interlinked. 

The Dancy-Darwall definition might look useful and attractive to you if you, like me, think that moral obligations obtain in virtue of reasons, and if you think that the reasons that give rise to moral obligations are themselves not moral phenomena and have their status as reasons in a way that is prior to, and independent of, morality.

The TMU definition might look good to you if you believe that moral principles or other sorts of moral facts can be among the factors that turn facts into favorers.

I wonder if there is a way of defining 'moral reason' that will be both useful and substantially neutral about how these different regions of normative reality are interlinked. I suspect there isn't. I suspect that we need to make assumptions about how the moral realm is organized in order to have a useful definition of this particular term.