Only say 'rational' when you can't eliminate the word

In the limit, only speak when you cannot remain silent.

When discussing these third-person as you are now, cognitive algorithms as algorithms are being invoked. But we all know that "p" and "Alice thinks that p" are hardly reducible to each other, it's first-person items like "I believe that p" that are deflationary. So while it is clearly the case that you can imagine situations where the sky is not blue but it would be epistemically rational to believe that it is, that does not demonstrate situations where one could justifiably claim only one of "the sky is blue" and "it is rational to believe that the sky is blue" (indeed the justifiability of the former just is the content of the latter.)

I think the intended meaning is as follows:

Similarly, you can eliminate the [word] 'rational' from almost any sentence [you utter]. [Saying] "It's rational to believe the sky is blue", [saying] "It's true that the sky is blue", and [saying] "The sky is blue", all convey exactly the same information about what color you think the sky is - no more, no less.

As you pointed out, the first sentence is not logically equivalent to the second and third (the second and third are logically equivalent according to Tarski's semantic theory of truth).

Alternately, if the sky IS blue, and someone objects to jumping to that conclusion, you can point out that the obvious conclusion is in fact rational in addition to claiming that it's correct.

"the sky is blue" and "it is rational to believe that the sky is blue" (indeed the justifiability of the former just is the content of the latter.)

This. You have created an example that shows that it is utterly impossible for a creature with our limited primate capabilities to actually know The Truth. What we can do is pay close attention to what we think and why we think that to be so.

In your case, there's an overwhelming amount of good reason to believe the sky is blue to anyone who doesn't know about these loki-like aliens (I really wanted to call them 'lokiens'). You might be wrong, and it could possibly lead to bad consequences in the future. But the alternative, believing something to be true without good reason, is crazy.

I'd say you need it not just because it's a default assumption, but because it's also an inaccurate assumption. Sort of like how you specify that an inclined plane is frictionless and in a vacuum, but you don't mention that it's under the force of gravity, or that it will force away any object that would otherwise pass through it.

I guess it is an attempt to top off all the recent "a bit more about r-word" discussions and move one already.

I think that both those are actually great examples of where using a word like "rational" obscures rather than clarifies.

What about saying "it is rational to feel X"

We have less direct control over our feelings than our explicit thoughts. It may or may not be irrational to endorse a feeling, but I for one think of feelings as neither rational nor irrational, but mere facts, and I think that's the common usage. You'd likely be better off thinking, "Y is true, but feeling X is inconsistent with my goals if Y is true. Therefore I want to feel less X."

For example, let's say that I am afraid of the dark, but it does not in fact achieve my goals to avoid the dark, or to be afraid when it's dark. Then what I want to do is notice that the feeling is unhelpful, and take actions to reduce it.

What does it add to say that the feeling itself is irrational?

What about the actions of someone else? "It is rational to not eat the cake" vs. "don't eat the cake"?

Here, "rational" is substituting for a claim about whether eating the cake has some specific effect, or optimizes or fails to optimize some goal or utility function. It is more precise to make the claim explicit than to use a vague term like "rational." For example, you might say that it is unfair for them to eat the cake, or that they probably wouldn't be better off with the extra calories, etc.

Anything from "The best X" to "optimal X" to simply stating your opinion. The normal assumption around here is that you're trying to be rational to the best of your ability.

My take on it is - "rationality" isn't the point. Don't try to do things "rationally" (as though it's a separate thing), try to do them right.

It's actually something we see with the nuts that occasionally show up here - they're obsessed with the notion of rationality as a concrete process or something, insisting (e.g.) that we don't need to look at the experimental evidence for a theory if it is "obviously false when subjected to rational thought", or that it's bad to be "too rational".

For me it's this: From a pragmatics perspective "the rational way to buy a car is..." repeats information-- when a person shares a method of doing something everyone assumes the speaker thinks that method is rational. Repeating it is redundant and redundant speech acts have a tendency to come off as arrogant and squicky. It's what you do when you talk down to someone.

It's also just sloppy to use words with connotations that don't apply when a better word exists. "Rational" connotes some general discussion of cognitive algorithms.

So I suspect it's a combination of a)sloppiness is bad and b)sloppiness looks and sounds bad

I think the objection to rational stems largely from this. Rationalism has a negative connotation in society thanks to, among other things, Hollywood and Ayn Rand.

See also: Straw Vulcan

Optimal

Let's try "Bayesian".

This is seriously false.

I think your claim as stated is literally false, but I completely agree with your sentiment.

This is among the highest-voted posts ever and doesn't use the word “rational”.