The first thing I found interesting was how the fact that truth conditional semantics can make valid arguments which directly contradicts non-cognitivism where they state that moral language is only opinions, commands etc.
Second, because we know that moral language can establish truth claims based off of valid arguments, it doesn't follow that moral language is full of simply opinions. Furthermore, the non-cognitivist view that moral language is just opinions can't form valid arguments at all; "what you freely assert, I freely dismiss."
I find it absolutely fascinating that an entire theory of philosophy can be obliterated by one argument. It really stakes the claim that truth can be found through rationality (assuming this is all true that is which it does seem likely).
So what do you guys think?
Just a lay person, but... I think it's odd that the theory was "destroyed" by an argument using formal logic. It presupposes that a) humans are logical b) they act in a logical fashion c) if stealing is bad then murder is bad is not an opinion, and d) opinions can't be logically consistent (wtf?) A 'logical' argument can be made for being homophobic: non-procreation is bad; hence, homosexuality is bad. Yet, a) not everyone who is homophobic uses arguments, because a bunch also just want to follow scripture, b) this can still be an opinion [1] and logically consistent. What happens in the real world, though, is that non-procreating/infertile couples do not face discrimination in the same way [2], and a homosexual might procreate and then raise the baby with a partner, and still face discrimination. This does not explain cultural differences, and exceptions, and other arbitrary moral attributes (i.e. not 'logical' ones). If I can find a cross-cultural counter-example for something one culture deems moral, and both are equally logical in their claim, what does that say about logic? That is, to go meta-meta: why is something being logical good? Examples: if murder is bad, how are capital punishment and war good? How is a crime of passion excusable? Most importantly, how is being pious a marker for morality? If my culture thinks maximizing self-interest leads to maximum aggregate happiness, and another culture believes it comes from maximizing group interest, both are logically consistent arguments in and of themselves. There is a (now famous) classroom experiment where the teacher said that blue/brown eyes were superior [3], resulting in kids treating each other in a generally shitty way. What I mean is: a) morality is tied to power, and that b) it is perfectly capable of producing a logical system that can be immoral. Take racism as an example, take the moral judgements of colonists as an example. Notes:
[1] Why is non-procreation bad? And have attitudes about the morality of an act changed? Does that mean a change of logic or of opinion? [2]As an aside -- procreating has a high value placed on it in the Old Testament, for example, and G-d often promises prophets 'a nation', because there is no concept of heaven. Hence, this system is consistent in placing a value upon procreation. [3] https://en.wikipedia.org/wiki/Jane_Elliott#The_first_exercise_using_brown_collars See also: Stanford Prison Experiment TL;DR: 
I'll try and take this one at a time I don't see what humans being logical has to do with the conclusions logic comes to. Using logic doesn't assume humans are completely logical. This also doesn't have to do with the conclusions logic comes to. Here you're confusing a valid argument from a sound argument. A valid argument is one where the conclusions necessarily follow from the premises, true or not. A sound argument is a valid one where the premises are true. Here the video was using a sound argument to prove that moral language has logical properties. Not that the example he used is necessarily true. All he's showing is that moral language has logical properties. What's really being said here is that non-cognitivists are saying that "Murder is bad" is really "I disapprove of murder." Sure you can make logical arguments such as: I don't do things I disapprove of; i disapprove of murder; therefore I don't murder. But the issue here is that those arguments aren't of any use within the field of ethics. If you were a non-cognitivist, you can't create a valid argument when you interject "I disapprove of murder" in the middle, since it doesn't necessitate any conclusions valuable within the field. Again, this is the different between a valid and a sound argument. You listed a perfectly valid argument; the premise, however, might not be (and most likely isn't, but that's another discussion) correct. Back to your earlier point here: humans being and using logic doesn't mean certain conclusions are right or wrong. Furthermore, I think they do use an argument without consciously admitting it: I believe the scripture is infallible; scripture says gays are bad; therefore gay's shouldn't marry You're right if you interpret "opinion" as "I believe these first principle are correct" I hate to sound redundant here, as I'm sure this must be irritating to read but this comes down to a difference between first assumptions. The arguments can be valid, just not sound. Now that's a good point. There's a whole field dedicated to answering this question (i.e. logic) but my simple answer would be this: Since we're in the field of seeking truth, and there's two essential ways of seeking truth (empiricism and reason (science and logic)) then an answer is good inso far as it uses those methods. Did we define the goalpost here (i.e. Seeking truth through those methods)? Yes, but that's the best I can come up with considering I'm severely under qualified to answer this enormous question. I'd have to give it more thought, but it's interesting to think about! If you haven't already, read some works by Nietzsche, you would like him a lot. But back to the point at hand here: it's one thing to say that humans act because of power, and it's another to say that humans should act because of power (look into the fact/value gap). What you're talking about is psychology of morality or why we act. But again, what we do is not necessarily the way we should act. Also, this is assuming that all moral actions (that is, all actions) are based around power. I would argue that all actions are based around happiness or wellbeing and power is just a means to that end.I think it's odd that the theory was "destroyed" by an argument using formal logic. It presupposes that a) humans are logical
b) they act in a logical fashion
c) if stealing is bad then murder is bad is not an opinion
d) opinions can't be logically consistent (wtf?)
A 'logical' argument can be made for being homophobic: non-procreation is bad; hence, homosexuality is bad
not everyone who is homophobic uses arguments, because a bunch also just want to follow scripture
this can still be an opinion [1] and logically consistent.
If I can find a cross-cultural counter-example for something one culture deems moral, and both are equally logical in their claim, what does that say about logic?
That is, to go meta-meta: why is something being logical good?
morality is tied to power, and that b) it is perfectly capable of producing a logical system that can be immoral.
Thanks for the great comment! I'm a bit more on the applied side of things (social sciences), so I consider that (application) more than things philosophers consider, which you have outlined in your comment. My approach is more concerned with how people view, act on, and use morality, which is why my comment reads the way it does. I will read Nietzsche sometime, I've heard good things :) Again, thank you for taking out the time to write a play-by-play explanation/rebuttal!
I'm glad I could help! But there's no need to thank you; I just like sharing knowledge with others about things I'm passionate about. When you have a chance read this page on Egoism. It has to do with psychology of actions so it's a perfect fit. Let me know if you have any other questions in the future and I would be glad to help with what I can.