I won’t retread well-argued ground about two seemingly contradictory words that actually mean the same thing. In fact, the OED lists the definition of flammable simply as “=inflammable.”
Linguistics argues that if two words are pure synonyms, then one of them will cease to exist. The fact that so many synonyms exist is because their are semantic differences in the meaning, one’s that do not necessarily show up in the definition but are there in what the use of the word implies. For example, a chat and a conversation may fundamentally mean the same thing, but you would be less likely to say “I had a chat with my attorney the other day…”
In this case, though, I can see no semantic difference between the two terms either. In fact, according to the OED, flammable had almost disappeared from modern usage as linguistics would have predicted, but the phrase was brought back to remove confusion on safety notices. See Simpsons clip above…
I have a proposal then. For purposes of setting things on fire, let us only use the word flammable. It is clear, to the point, and no one will put open flames on their mattresses because “hey, it said inflammable so I’m safe right?”
Inflammable should still be used, but not for fire. Rather, it should be used for its secondary definition: Easily roused to excitement. After all, the word really comes from inflame, which carries far more meanings than just “able to burn.”
In conclusion, the bed is flammable, a personality is inflammable, and quick-to-anger person doused in karosene is both.