Words can become less useful when they attach to too much as well as too little. A perfectly drawn map that indicates only the position and exact shape of North America will often be less useful than a less-accurate map that gives the approximate location of its major roads and cities. Similarly, a very clearly drawn map that does not correspond to the territory it describes is useless. So defining terms clearly is only one part of the battle in crafting good arguments; you also need terms that map well onto the actual territory and that do so at a useful level of generality.

The problem with the term "rationality" isn't that no one knows what it means; there seems to be wide agreement on a number of tokens of rational behavior and a number of tokens if irrational behavior. Rather, the problem is that the term is so unspecific and so emotionally loaded that it obstructs rather than furthers discussion.

Imagining that someone "had a reason to seriously present" to Obama-Mammoth hypothesis is to make the hypothesis non-absurd. If there is real evidence in favor of the hypothesis, than it is obviously worth considering. But that is just to fight the example; it doesn't tell us much about the actual line between absurd claims and claims that are worth considering.

In the world we actually inhabit, an individual who believed that they had good reasons to think that the president was an extinct quadruped would obviously be suffering from a thought disorder. It might be interesting to listen to such a person talk (or to hear a joke that begins with the O-M Hypo), but that doesn't mean that the claim is worth considering seriously.

Christianity is false, but it is harder to falsify it then it is to show that Barrack Obama is not a non-sapient extinct mammal. I can prove the second false to a five-year-old of average intelligence by showing a picture of Obama and an artist's rendition of a mammoth. It would take some time to explain to the same five-year-old child why Christianity does not make sense as a description of the world.

This difference—that while both claims are false, one claim is much more obviously false than the other—explains why Christianity has many adherents but the Obama-Mammoth hypothesis does not. And we can usually infer from the fact that many people believe a proposition that it is not transparently false, making it more reasonable to investigate a bit before rejecting it.

Not necessarily. The vast majority of propositions are false. Most of them are obviously false; we don't need to spend much mental energy to reject the hypothesis that "Barack Obama is a wooly mammoth," or "the moon is made of butternut squash." "Absurd" is a useful label for statements that we can reject with minimal mental effort. And it makes sense that we refuse to consider most such statements; our mental time and energy is very limited, and if we want to live productive lives, we have to focus on things that have some decent probability of being true.

The problem is not denominating certain things as absurd, it is rejecting claims that we have reason to take more seriously. Both evolution and Christianity are believed by large enough communities that we should not reject either claim as "absurd." Rather, when many people believe something, we should attend to the best arguments in favor of those beliefs before we decide whether we disagree.

The fact that you do not value something does not serve very well as an argument for why others should stop valuing it. For those of us who do experience a conflict between a desire to deter and a desire to punish fairly, you have not explained why we should prioritize the first goal over the second when trying to reduce this conflict.

On this we agree. If we have 60% confidence that a statement is correct, we would be misleading others if we asserted that it was true in a way that signalled a much higher confidence. Our own beliefs are evidence for others, and we should be careful not to communicate false evidence.

Stripped down to essentials, Eliezer is asking you to assert that God exists with more confidence than it sounds like you have. You are not willing to say it without weasel words because to do so would be to express more certainty than you actually have. Is that right?

There seems to be some confusion here concerning authority. I have the authority to say "I like the color green." It would not make sense for me to say "I believe I like the color green" because I have first-hand knowledge concerning my own likes and dislikes and I'm sufficiently confident in my own mental capacities to determine whether or not I'm deceiving myself concerning so simple a matter as my favorite color.

I do not have the authority to say, "Jane likes the color green." I may know Jane quite well, and the probability of my statement being accurate may be quite high, but my saying it is so does not make it so.

You do not cause yourself to like the color green merely by saying that you do. You are describing yourself, but the act of description does not make the description correct. You could speak falsely, but doing so would not change your preferences as to color.

There are some sentence-types that correspond to your concept of "authority." If I accept your offer to create a contract by saying, "we have a contract," I have in fact made is so by speaking. Likewise, "I now pronounce you man and wife." See J.L. Austin's "How to Do Things With Words" for more examples of this. The philosophy of language term for talking like this is that you are making a "performative utterance," because by speaking you are in fact performing an act, rather than merely describing the world.

But our speech conventions do not require us to speak performatively in order to make flat assertions. If it is raining outside, I can say, "it is raining," even though my saying so doesn't make it so. I think the mistake you are making is in assuming that we cannot assert that something is true unless we are 100% confident in our assertion.

Sure, it is useful to ask for clarification when we don't understand what someone is saying. But we don't need to settle on one "correct" meaning of the term in order to accomplish this. We can just recognize that the word is used to refer to a combination of characteristics that cognitive activity might possess. I.e. "rationality" usually refers to thinking that is correct, clear, justified by available evidence, free of logical errors, non-circular, and goal-promoting. Sometimes this general sense may not be specific enough, particularly where different aspects of rationality conflict with each other. But then we should use other words, not seek to make rationality into a different concept.