What's a Thesaurus?

I had an interesting conversation with a German native the other day, trying to explain what a thesaurus was.

Me:  Do Germans have thesauri?  I was trying to find one.
Her:  Do we have what?
Me:  A thesaurus.  Do you know of a good thesaurus for German?
Her:  What is that?
Me:  It's a book where you can find other words that mean the same thing as the word you know.
Her:  We have synonym books, if that's what you mean?
Me:  Sort of, but I was hoping for a good, proper thesaurus.  It's got more depth than synonyms - it breaks ideas down into chunks and gives you words that match those chunks.
Her:  Um... What?
Me:  Like, it would take the idea of furniture, break that into different kinds of furniture - like furniture you sit on - and then give you words for furniture you sit on.  Couch, divan, chaise-longue, chair.  That sort of thing.
Her:  Why on earth would anyone need that?

A good thesaurus - a true thesaurus - defines the universe.  It divides and subdivides all of the concepts within human experience and then offers you a vocabulary to use for those concepts.

It's more than a book of synonyms.  It's more than a dictionary, really - for it gives you the definition and then lets you find all the words that fit.  Well, many of the words that fit.  There are a lot of words in the world.

When I was in high school, Roget's Thesaurus was on the book list.  It became one of my most cherished tools.  I cannot imagine a world where this book does not exist.

As a librarian, I encounter other thesauri all the time.  The systems of subject headings (like MeSH) used by databases to classify and organise their entries are thesauri.  Whether the topic you are dealing with is the English language or the fields of medical knowledge, the point of a thesaurus is to ferret out the facets of a thing and give you a term you can use for any particular facet.

So, needless to say, I was absolutely flabbergasted recently when I read something that suggested English is unique in the word for having a thesaurus like Roget's.

Surely such a thing would be vital for any serious study of any language?  How could you take your language seriously and not have a thesaurus?  More so than a dictionary, a thesaurus maps the conceptual world of a language.  A dictionary will tell you the definition of a "divan" - a thesaurus will show you where the word fits into the grand scheme of things.

And, from a language learning point of view, a thesaurus would surely be the most useful of tools?  Rather than plowing through multiple dictionaries, trying to find the word that sort-of-maybe-kind-of matches the word you have in mind - constantly finding words that don't quite match up - you could just look up the definition of the word you want, and find the words that match that definition.

I have a book at home which is a multilingual exploded visual dictionary, and it is the most marvelous thing.  It's sort of, kind of, almost a multilingual thesaurus, only there are pictures involved.  You can find a word for the hilt of a sword in six language just by finding the picture of a sword and looking for the hilt.

That's what a multilingual thesaurus should be.  That's what it can do.

That's a powerful and useful thing.

So, applied linguists of the world, get cracking.  There's the whole of human language to map.