Often the prefix 'in' functions as a negative. But not always. For example, 'combustible' means 'capable of catching fire' and 'incombustible' means 'made of material that does not catch fire'. On the other hand, both 'flammable' and 'inflammable' which appear to be antonyms, actually mean the same-easily set on fire. The prefix 'in' here serves as an intensifier. So is the case with habitable and inhabitable; debted and indebted. The opposite of 'flammable' is 'not flammable', and that of 'inflammable' is 'not inflammable'.