All symbolic communication is learned, negotiated, and dynamic. We know that the letters b-o-o-k refer to a bound object with multiple written pages. We also know that the letters t-r-u-c-k refer to a vehicle with a bed in the back for hauling things. But if we learned in school that the letters t-r-u-c-k referred to a bound object with written pages and b-o-o-k referred to a vehicle with a bed in the back, then that would make just as much sense, because the letters don’t actually refer to the object and the word itself only has the meaning that we assign to it. We will learn more, in Chapter 8 “Verbal Communication”, about how language works, but communication is more than the words we use.
It's interesting how, for those who grew up with English as their native language, the symbol for a big brown thing sticking out of the ground with green puffs is recognized as a 'tree.' In other languages, the symbols or words might be similar or completely different. As someone who is bilingual, I've noticed that the word for 'tree' in our second language is significantly different from the English term. If we were to integrate that word into the English language, it would have no meaning at all.