Communication is Hard
Some people assume communication is automatic. You say something, and the other guy knows what you mean. No thought is required. No error is possible, or at least it's extremely unlikely. Words have established meanings, which everyone knows, and if anyone isn't sure then the dictionary will give a definitive answer.
This is false. Communication is hard.
Imagine communicating with an alien. Even deciding what is intended to be a message, and what is meaningless, is tricky. The alien observes us. We make various noises. Some from our mouth, such as words and yawns. Some from our feet, such as footsteps. We make various movements, such as pointing at things, walking and swaying in the wind. We emit various odors. We reflect various frequencies of light (colors), and adorn ourselves with garments which reflect different colors. How is an alien to know which of these things contain messages and which don't? It's not obvious.
If the alien figures out which things are messages, how is he to know what they mean? Suppose you say a word and point at something. How does he know that pointing means to associate a word with an object? He doesn't. How does he know that when you put your hand in a certain configuration he's supposed to draw a line from the base of your index finger to the tip, then continue it through space, until he finds the first thing you would consider notable enough to point at? He doesn't.
This is what I call the communication gap. There is a gap between my mind and the alien's mind. The gap consists of lack of shared knowledge, lack of shared assumptions, lack of background knowledge, and so on. And as a result, communicating is hard. It's hard to tell what is a communication, and hard to tell what communications mean. But it is possible for communications to cross the gap, and for beings who are very different to come to understand each other.
The way communications bridge the gap is the same way all knowledge is created. We can phrase the problem like this: I want to create knowledge of which things from the alien are messages, and what they mean. I do this by imaginative conjectures and criticism of those conjectures to eliminate errors. Because this is the only known way to create knowledge, it's also the only known way to bridge the communication gap.
All communication involves active knowledge creation by the participants which is how the difficulty of communication can be overcome.
In more detail, what I need to do is make guesses about which of the things the alien does are intended to communicate, and which have no meaning. And then I need to think about whether the guesses make sense. And I can try out the guesses: assume (perhaps only in a thought experiment) that the guess is true, then continue from that assumption and see if it works out. As I work on this, I can get feedback from the alien who will be trying to figure out what I am saying. If the alien isn't trying to help, or if I'm not making a serious effort, then we may never understand each other, but as long as we try we can make progress. After some wild guesses, we might hit on something that works a little bit, and get to the point of understanding "yes" and "no", so at least we can say if the other guy is on the right track. From there, things will improve as our shared knowledge increases, and eventually the alien could understand the entire English language.
Even once we have a shared language, our problems aren't all gone. Consider speaking with a friend. No English statement is ever perfectly unambiguous, nor does any English statement ever perfectly match every nuance of the ideas in my head. Of the possible meanings I might intend, how is my friend to decide which I did intend? He will have to create knowledge of what I am trying to communicate. It's the same communication gap as with the alien, easier due to shared knowledge, but it still exists. You might say to ask my friend what he meant. That can help, but we can never be completely precise, so there will always be some judgment call on my part. This requirement that we interpret and judge is not a problem or a bad thing, it's simply a fact which we are already accustomed to living with every day.
A shared language does not actually mean everyone has the same understanding of all words. People are different, and there are different nuances to how they think, how extensive their vocabulary is, their understanding of the connotations of words, and sometimes people even have different understandings of common words. For controversial words, differences in understanding of them are ubiquitous.
No two people are exactly the same. Often people's understandings of words are close enough that, for many purposes the differences are negligible. But sometimes the differences are relevant and this causes misunderstandings. The only way to avoid such misunderstandings is to think about how other people understand words — to create knowledge about what they are trying to say.
Misunderstandings may seem pretty uncommon, but that's only because people aren't vigilant enough in looking for them. Many misunderstandings go unnoticed. Often people think they agree, and never discuss the details enough to find out if they really do agree, or not. Sometimes they disagree but they interpreted the same statement to mean different things. The only way to find out if you agree with someone is to be aware that communication is difficult, watch out for possible misunderstandings, discuss some details, and when there is a hint of disagreement pursue it. In short, our tool against misunderstandings is the methods of error correction, whereas assuming there won't be misunderstandings is a way to avoid error correction and thus guarantee misunderstandings.
We began with aliens, which do not have a shared language, but moved on to see how discussions between Americans also face a smaller — but still substantial — communication gap. Now let's consider children: infants are not born knowing our language. They aren't born familiar with our culture, sharing our customs, or with any other head start that an alien doesn't have. Infants face the same communication gap to learn English that an alien does.
In fact, infants may well have a harder time than aliens. If the aliens have invented space ships, then they have extensive knowledge. They know math, which we know too. They know some physics and chemistry and astronomy, which we know too. That is shared knowledge. And they have computers which they can use to automate things to help them out, whereas infants don't know how to use computers.
In some ways, it's amazing that anyone ever learns English. It's a hard problem to bridge the communication gap. It's a hard problem to guess what various sounds mean, with no quick way to find out which guesses are correct, and then to eliminate errors in those guesses over time. It's a bit like cracking the code of an encrypted communication. But infants reliably succeed at it. That means if we do encounter intelligent aliens, we can expect they will be able to succeed at understanding us too, and we them.
There are two major lessons here. One is about infants: they are able to create knowledge. They have fully functional brains capable of solving difficult, abstract problems. They may be ignorant of numerous things, but the primary ability of the mind — to create knowledge — works. This has some consequences for parenting philosophies which assert children are stupid, or unable to think for themselves, or unable to think abstractly, etc...
For the second lesson, here is a true version of the introductory paragraph:
Some people assume communication is automatic, but it isn't. When you say something, the other guy has to guess what you mean. Thought is required and error is possible. For people with shared background knowledge such as the English language, they have a useful advantage that helps them communicate well, but miscommunication remains common. Words have established meanings in the dictionary, which everyone knows slightly differently because we can never be completely precise, and because people are different and think differently, at least in subtle ways.
The key to communication, as to many things in life, is knowledge creation and error correction.