Why is probible not possible and possable not probable?

In English when we want to describe something as “capable of doing something”, there is a handy set of adjectives that we can use which end in either -able or -ible. Examples are numerous, to name a few, we have adorable, applicable, curable, observable, operable, portable, probable for -able, and divisible, edible, feasible, legible, permissible, plausible, possible for -ible.

These two suffixes carry exactly the same meaning, i.e. “capable of V-ing”, or more often “capable of being V-ed”, where V is a verb constituting the stem of the word. For instance, applicable describes something that is “capable of being applied, and changeable describes something that is “capable of changing/being changed”. They are, however, not interchangeable, as illustrated by the non-existence of the words *probible and *possable. Why, then, do some words use -able and others -ible?

If you look at the above list of words long enough, it may seem that a rule emerges if we pay attention to the letter just before the suffix:

able -ible
adorable divisible
applicable edible
curable feasible
observable legible
operable permissible
portable plausible
probable possible

One might reasonably hypothesize that if the stem ends in certain letters (b, c, r, t, v), -able is used, and if it ends in certain other letters (d, g, s), -ible is used. Such claim can however be easily rejected once we consider more examples:

able -ible
affordable collectible
excusable convertible
formidable horrible
laudable invincible
navigable reducible
subjugable susceptible
usable terrible

This shows, in fact, most of the consonant letters can be followed by both -able and -ible. In other words, the distribution of these two suffixes is not phonologically conditioned.

To understand the distribution, we have to trace the history of these suffixes. It should not be difficult to guess that -able and -ible have their origin in Latin, from the suffixes -abilis and -ibilis respectively. As in English, they are attached to the end of verbs to form adjectives. In Latin, verbs are grouped into different classes, some end in -āre, some in -ere, and still some in other endings. Let us break down the English words above and take a look at their Latin origins:

English Latin origin
word stem meaning suffix
adorable adōrāre beseech -abilis
applicable applicāre apply -abilis
curable cūrāre care -abilis
observable observāre observe -abilis
operable operāre work -abilis
portable portāre carry -abilis
probable probāre try -abilis
divisible dīvidere divide -ibilis
edible edere eat -ibilis
feasible facere do -ibilis
legible legere read -ibilis
permissible permittere let go though -ibilis
plausible plaudere applaud -ibilis
possible posse be able -ibilis

Now it should be clear that -abilis is added to Latin verbs which end in -āre, and -ibilis to verbs that end in -ere and other endings. This was the rule applied in Latin when the suffixes were first used. However, such distinction was abandoned and only -able was used when new words were coined in Old and Middle French.

Words such as possible and edible were earlier creations that existed in Latin already, so they followed the above scheme and used the -ible. Later French creations, such as tenable (from O.Fr. tenir, from L. tenēre) and attainable (from O.Fr. ataindre, from L. attingere) no longer followed this rule and used -able instead. In later development, the suffix -able was further generalized in English to apply to all native English words such as break (breakable), stop (stoppable) and forget (forgetable).