Rationality is something that we do not always understand. Inquiry is something that everyone uses at some point or other. But rationality is more than grammatically correct form and some combination of truthfulness or truth-aptness.
Rationality is a presupposition of all inquiry. In other words, every question presupposes that there is a rational answer to that question. If I ask “where are you?” I am presupposing that you exist and that you are present at some location or other. This means that you are not an abstract object, you are not omnipresent and there is such a thing as “place”. But this is true for absolutely every question that can be constructed.
Begging the question is irrational in all contexts and in all circumstances. Begging the question (also known as circular reasoning) covers a wide variety of arguments. What all of these arguments have in common is that the conclusion of the argument is presupposed by the argument. There are a number of ways to do this. First, it might be identical to one of the premises. Second, it might be identical to the logical form of the argument. Or much less commonly, it might be identical to a concept found in one of the premises. No matter which form it takes, the problem with begging the question is that the argument is an illusion. Begging the question proves nothing but gives the appearance of proving something. Circular reasoning is a way to trick someone into believing something rather than convince them by rational means.
When both of these facts are considered together, we should wonder whether some questions might beg the question simply by their nature. Consider the question “Is the universe rationally ordered?” Since this is a question, it presumes that there is a rational answer to that question. But anyone asking the question is a part of the universe and therefore covered by the question. As a result, any attempt to answer the question will be an attempt to rationally answer the question of whether or not the universe is rational. But this is an instance of begging the question. Therefore, any attempt to answer the question will also be irrational. Since rationality is the presupposition of inquiry, there are some questions that cannot be rationally answered.
Consider the liar sentences. One such sentence is that “this sentence is false”. If the sentence is true, then it is false. Therefore it cannot be true. But if it is false, then it is true. Therefore, it cannot be false. But this means that the sentence cannot be true or false. Since we normally consider any meaningful indicative statement to be either true or false, we are forced to conclude either that some meaningful statements are neither true nor false or that some apparently meaningful statements are not really meaningful at all.
It might seem that any meaningful question has a rational answer, but I have already shown that some meaningful questions cannot have a rational answer because they cannot be rationally asked.
What this means is that rationality and meaningfulness cannot be considered as elements of an equation. It is not as if we can begin with properly formed (grammatically correct) indicative sentences and then follow a mechanical procedure to determine which ones are true and which are false. No such procedure will ever work. I have shown this by indicating that some sentences are grammatically correct, in indicative form but are neither true nor false. I have also shown this by showing that some sentences can be mentioned, are in a grammatically correct interrogative form but cannot be rationally asked (used). In simple terms, rationally and meaningfulness are distinct but more grammatically correct form and truthfulness.