Flammable and inflammable

It is crazy, impolite means ''not polite'', but imflammable does not mean ''not flammable''. both flammable and inflammable mean the same thing, isn't that crazy?

"Inflammable" is the original word picked up from French and has always meant "liable to catch fire". The French world itself orignated from the latin verb "inflammare" (to catch fire). At some stage in the English language evolution, some people may have begin to simplify it by saying just "flammable". So it is not the last which took a suffix, it is Inflammable which was shortened.
In French no such a shortcut exists.

The same with valuable and invaluable.

If inflammable and flammable mean the same thing, then why don't polite and impolite mean the same thing.

The same with valuable and invaluable.

My dear Richard,
English is full of irregularities and exceptions.
English is a language of exceptions 'à merveille'.

The "in" in "inflammable" isn't a negative prefixe, it's just a part of the word. I understand that people may not find it logical, but it is not the only illogism English has.

You have more of these in English :

Personal / impersonal
Perfect / imperfect
mature / immature

Does anyone know more adjectives like these ?

I have keyboarded something improperly in my last message. As everybody will have guessed, I meant "The French WORD originated from the Latin verb 'inflammare' " and not "The French WORLD".

Pooh, I think you're missing the point. We are talking about words which look like they have a negative suffixe, like the ones you wrote, but which actually have not (contrary to the one you wrote).

"Impersonal" means "not personal"
while
"Inflammable" and "flammable" means the same thing. See?

I think it would be more interesting to check if there are some other English words whose negative suffix are not negative suffix.
The words who have negative suffix are too numerous and often used to be dedicated a topic, in my humble opinion.

Each time I keyboarded "suffix" in the previous message, I meant "prefix".

´Inflammable´ comes from ´in+flammare´= to set on fire.

Flammare per se means set on fire, and it is no bad thing to have 2 or more verbs for the same action.

The difference comes out when you try to say the adjective form: Flammatus is only a verb, but Inflammatus is an adjective.

The good thing about these oddities is that they make you think.

What about valuable and invaluable ? Don't mean both almost the same thing ?

No, invaluable means you cannot attach a value to it, ie. it is worth so much.

As a 7 yr old, I couldn't understand why "priceless" didn't mean the same as free.

So priceless and invaluable are the same ?