If you’re trying to convince me to do some thing X, then you must want me to do X, too. So we must be at least that aligned.
We don’t have to be aligned in every regard. And you needn’t yourself value every consequence of X that you hold up to me to entice me to X. But you do have to understand me well enough to know that I find that consequence enticing.
But that seems to me to be both plausible and enough to support the kind of dialectical moral argumentation that I’m talking about.
It’s true that a moral realist could always bridge the is–ought gap by the simple expedient of converting every statement of the form “I ought to X” to “Objectively and factually, X is what I ought to do”.
But that is not enough for Sam’s purposes. It’s not enough for him that every moral claim is or is not the case. It’s not enough that moral claims are matters of fact. He wants them to be matters of scientific fact.
On my reading, what he means by that is the following: When you are pursuing a moral inquiry, you are already a moral agent who finds certain objective and scientifically determinable facts to be motivating (inducing of pursuit or avoidance). You are, as Eliezer puts it, “created already in motion”. Your inquiry, therefore, is properly restricted just to determining which scientific “is” statements are true and which are false. In that sense, moral inquiry reduces entirely to matters of scientific fact. This is the dialectical-argumentation point of view.
But his interlocutors misread him to be saying that every scientifically competent agent should find the same objective facts to be motivating. In other words, all such agents should [edit: I should have said “would”] feel compelled to act according to the same moral axioms. This is what “bridging the is–ought gap” would mean if you confined yourself to the logical-argumentation framework. But it’s not what Sam is claiming to have shown.