In Orwell's 1984, one of the totalitarian state's means of enslaving the populace was to outlaw many of the subtlties of language, and by restricting the common vocabulary so to restrict the ability to conceive dangerous thoughts. I think this as a method is worth considering in response to your question. I agree that without a word to name a mental concept, that concept is itself far more difficult, and impossible to communicate.

But the point about popular conversation, dominated as you say by slang, short-cuts and mis-nomers, is that it changes over time as word fashions mutate. I'm not sure its correct to say the common vocabulary decreases, and neither does its fitness for purpose. I'd guess it is limited by human brain size, so new words come in as fast as old ones are lost and it evolves to suit the needs of everyday life.

However, surely rigorous intellectual discussion stays above popular terms - it has to as common words are often insufficient to describe its concepts, and slang is irrelevant. How much has the language of philosophy or physics degenerated over time? Not much I'd guess..

Re the decline of the Classics: did the common peasant ever quote Virgil or Homer? I very much doubt it. Education is what expands our vocabularies, and the further we progress the more specific our terms become. Fewer of us these days are educated in the Classics than say 150 years ago, but then no-one in Victorian times could understand discussions on computing or abstract art...

So maybe what we have is a mutating common vocabulary, plus many specialised sub-vocabularies, only 1 or 2 of which most of us can fully understand. These specialised areas are the conduits of complex ideas. They have always been unintelligible to the layman, but nevertheless they will surely survive as long as the fields of study they describe.

The human lexicon has surely grown since we were herders in Mesopotamia. As a result of word invention we are more informed, more intelligent than ever.

If any one person desires to specialize in a particular domain of knowledge, they will read. The more they read in that domain the larger their breadth of understanding, consequently the increase in their intelligence.

You can be intelligent here but an idiot there.

I don't know whether you can have ideas for which you have no words or not. It seems like having words for which there are no ideas. Empty. Zero...

More experiences, more interaction with others, more reading, more learning, more attention to what is going on in the world, etc. should increase the complexity of thinking as well as the complexity of one's vocabulary. Experience, of course, comes first. Then comes a term for the experience.

I recently read "Quantum Space by Douglas Philips. Sci Fi. Pretty good. Philips does a good job of explaining real (but very elementary) quantum theory (as far as it's needed for the plot) and an equally good job of explaining the fictional quantum theory that makes the story work. In order to do this, he has to introduce terms --almost all of the terms he used are real -- boson, quark, spin, top, strange, etc. What he says about string theory is bogus, but that's OK, this is fiction, after all. We don't have a "graviton particle" yet, but describing the would-be particle, using the term and defining it makes it possible for the reader to participate in the story. I thought his explanation of the Standard Model was convincing (but I know very little about physics).

So, the same thing happens in real life. You learn new things; maybe somebody at a bar explains how "social membranes" work. Social membranes is batted around the table at the bar over beer, and you understand that a social membrane is the set of rules and identity that apply to a given situation. Tony Soprano (TV series about a mafia boss) lives in several social membranes. His home is one membrane. His official work (garbage hauling) is a pseudo membrane. It is there just for cover. His real work (theft, murder, embezzlement, fraud, abuse, etc.) is a third membrane. The Bada Bing Club is another membrane. Tony tries very, very hard to keep his social membranes separate. It drives him crazy, which is why he is seeing a psychiatrist.

Make sense?

You can be intelligent here but an idiot there. — Josh Alfred

God. That's so very very very true.

I don't know. If we look at word etymology, even the most difficult concepts in <difficult subjects> tend to reduce to very basic concepts. A good dictionary should suffice to make my point. We can also see this with synonyms e.g. ''comprehend'' is synonymous with ''grasp''. So the difficult concept of comprehension is the simple act of holding something in your hand.

So, it could be said that we haven't made any more progress than our ancestors. We're still using body language so to speak.

On the other hand, we've managed to see the pattern that underlies nature and that is a big leap in mental capabilities.

The use of simple concepts to construct grand edifices of science, math, philosophy, etc. Is indeed an achievement.

Do words affect the mind so deeply that they can hinder or further the growth of mind? — Occidendum

Well, I consider language as equipment. It helps us do useful things - express emotion, exchange ideas, describe stuff, etc. - but, like everything else, it has its limitations and disadvantages. Some things are ineffable, some things are really hard to describe, etc. Some words (concepts) in one language can't be correctly translated into another, showing that language can hinder our understanding of the world.