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?

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