Human cognition is based in language but trying to give precise answers to some of your questions i have selected this one:

Do adults understand everything they say?

Of course not. For example I could say random combinations of letters or words. These may have meanings in different languages but I can pronounce them without knowing or understanding what they mean.

Another example: Since Darwin many text books and scientific articles in Biology and Evolution mention Natural Selection, but when you ask in the scientific debates what is natural selection, then nobody knows. Nobody can give a precise answer. Then everything gets confused and people starts insulting you.

You could see here for an example:

https://www.researchgate.net/post/Is_there_a_scientific_alternative_to_neo-Darwinism_for_understanding_biological_evolution?ch=reg&cp=re68_x_p2&pli=1&login=ecervant@usal.es#view=50bddd2ce4f0760f1f000007

That took me to ask this question:

https://www.researchgate.net/post/What_is_the_meaning_of_insults_in_a_scientific_debate1

As for the question "Can adults say everything they understand?" there is a french saying that responds quite well I think "Ce qui se concoit clairement s'énonce clairement', which means " What you clearly understand, you can clearly explain" and I think that unless you've got a speech disorder this is indeed the case. Maybe you cannot explain some things with words, but then you shouls still be able to explain it using gestures or drawings.

However, I do think that language is limitative and sometimes insufficient to explain the full width of a phenomenon.

When you say " It seems children can talk without understanding.", I wonder who says so? Do you have references for this?

Michele, this might be interesting, but my question was the opposite: Do adults understand what they say?

Well, I think when they say what they say with intent to communicate, be that with themselves or with the outside world then my answer would be yes. I think the crucial issue here is, that as long as they are conscious of what they are saying then they will understand themselves. Whether what they say is understood by others too, obviously depends on other factors (like background, intelligence, ...).

Leonid, Observation suggests that people only understand what they say to a limited extent. Often you get surprised when someone explains to you what you have just said. (You might reply: I didn't mean that! but that is irrelevant.) This is because any statement can be perceived from multiple perspectives. A person does not use all these perspectives simultaneously—which would be impossible. Understanding needs to be itself understood within a reference group and its conception of reality--you don't seem to give that attention.

Michel,

I agree that people are usually (to some extent) conscious of what they say. Does it mean they really understand it? Data shows that when speaking about abstract ideas (even moderately complex) most of people repeat phrases that they heard from others. They "understand" it in terms of language, but they do not understand it in terms of the real world. (It is known that abstract words are processed by language brain areas, but NOT by cognition brain areas).

Just listen to TV discussions about Global Warming. Most of commentators (supposedly more intelligent than "average" people) do not even differentiate between (i) does the Earth average temperature rise? and (ii) is it caused by human activity? Certainly none of them are climatologists who understand the physical processes involved. They just repeat what "scientists" presumably say. Even worth, they do not care about scientists and say what is supposed to be "politically correct" in their own circles.

Let me emphasize, I do not discuss here if these opinions are correct. I just emphasize that most people do not "really" understand what they say. And they are unconscious about this missing understanding. So are people really conscious of what they say?

This is a politically charged topic. But the same is true about most usual everyday conversations as soon as it goes beyond what people "really know" from their everyday experience, say how to fry eggs.

Warren,

It is an excellent comment, exactly what I just wrote above. Michele - please note!

People do not usually understand others - this is very interesting and related, but a different topic. "This is because any statement can be perceived from multiple perspectives." - I agree, but I'd like to emphasize that the actual case is even worth.

Of course "Understanding needs to be ... within a reference group and its conception of reality--you don't seem to give that attention." You are right, I do not yet consider this, because there is a deeper missing understanding. I emphasize that "understanding within a reference group and its conception of reality" is usually limited to language and is far from reality.

What would you say?

Reality is unknowable but we have to deal with it. Language is the method that we use. (Non-verbal processes of thinking etc are relevant but the final common pathway is language.)

So I think you have to grasp the nettle of my "methods of using language" to engage with your last sentence. I've not posted this, but some of the methods are very close to reality and are the basis for convictions and beliefs. For example, have you ever followed instructions to assemble flat-pack furniture? I would say that language is very close to reality--frustrating, but precise. The language of software manuals is similar.

The associative method regards reality as common-sense and it works most of the time when used in the appropriate time and place. My logical style of communicating uses beliefs/assumptions that feel like they correspond to reality (for most people!).

Another method of using language uses socially agreed concepts and then develops consensus in their group to specify reality: scientists talk like this. We know they are wrong most of the time, that outsiders who are right get rejected, and much of what is written is eventually found to be wrong....but it doesn't stop them getting grants and being treated as heros who unveil reality!!

In the TOP Studio, there is a page: Using Language > Seven Ways to Communicate > Confrontation with Reality. Much more detail and comparison on this important topic will be posted in due course. If you wanted to engage with real life, then you should look at the material on Work.