Why does "-Cy" become "-Sy"?

What rule of grammar, or etymological history, makes "prophe-cy" (noun) become "prophe-sy" (verb)? What causes the C to become an S when the word usage changes?

According to the Oxford English Dictionary, the differentiation between prophecy and prophesy as noun and verb, respectively, happened more or less arbitrarily; before circa 1700, both variants were used for both noun and verb each:

ME. a. OF. prophecier (1245 in Godef.), -phesier, -fecier, -ficier, f. prophecie, -fecie prophecy. The modern differentiation of prophesy vb. and prophecy n. was not established till after 1700, and has no etymological basis, prophesy being at first a mere spelling variant in both n. and vb.

The reason why this pair exists, while either c or s was chosen for both forms in other Greek words, is most probably confusion about which Greek word they were derived from. Normally, Greek -tia results in English -cy/cie, while Greek -sis results in English -sy/se/sis (both through French). This development is probably related to two facts:

• C and s have long been pronounced the same before y, so that they are easily confused. Vitaly has automatically compiled a list of words of Greek origin that have been found to occur with either spelling in major dictionaries; this illustrates how c and s were often interchangeable in the past. Note that most, if not all, of these words are now only spelled either -cy or -sy. Note also that one could no doubt find many more searching the Oxford English Dictionary for alternate spellings by hand.

acracy/acrasy, apostacy/apostasy, eustacy/eustasy, idiosyncracy/idiosyncrasy, isostacy/isostasy, prophecy/prophesy, syncracy/syncrasy, theocracy/theocrasy

• Most Latin and Greek words first came to us through French, in which -ti- was often softened to -ci-. The original Greek word was prophêteia, which would normally result in -c- in (Old) French; that is probably where prophecy came from. The -t- in Greek is there because the word is derived from prophêtes ("prophet").

But a more common way of forming a word with a similar meaning from the same verb stem (phê-) in Greek would be prophêsis (cf. haerêsis => heresy); from this, the French derivation would result in -sie, -se, or -sis (though -sis is rather modern, as it is the exact Latin transcription of the Greek word). Prophêsis did not exist in classical Greek, because their only word for prophecy and the action of prophesying was not derived directly from the verbal stem, but from the noun prophêtes as above. But ignorance of this fact is probably what led people to spell -s- sometimes.

Perhaps certain writers thought the s was closer to the Greek and preferred it, especially in the verb, because they though it was -t- in the Greek noun and -s- in the Greek verb (which is not true). Or perhaps the differentiation developed for even more arbitrary reasons.

These guidelines are applicable in British English:

When two words, verb and noun, are spelt alike, “se” and “sy” are verb endings; “ce” and “cy” are noun endings. The following are therefore verbs: license, practise, prophesy, advise.

The following are nouns: licence, practice, prophecy, advice

Promise is an exception to this rule. It is both verb and noun.

This rule does not hold good when verb and noun are not spelt alike, so suspense and hypocrisy are nouns.

In American English, only prophecy (verb), prophesy (noun), advise (verb), and advice (noun) follow this guideline, with licence and practise being considered British spellings.