Etymology of English suffix -logy

Question

I excerpt OED, which I read because I want to understand this etymology.

-logy, comb. form

... These Greek words for the most part are parasynthetic derivatives; in some instances the terminal element is λόγος word, discourse (e.g. in τετραλογία tetralogy, τριλογία trilogy); more commonly it is the root λογ- (ablaut-variant of λεγ-, λέγειν to speak: cf. Logos n.). In the latter case, the ns. in -λογία usually denote the character, action, or department of knowledge proper to the person who is described by an adj. or n. in -λόγος, meaning either ‘(one) who speaks (in a certain way)’, or ‘(one) who treats of (a certain subject)’. Hence the derivatives in -λογία are of two classes,
(1) those which have the sense of ‘saying or speaking’, ...; and
(2) names of sciences or departments of study.

I don't know any Greek. Would someone please explain how one word can evolve into the 2 separate meanings (that I bolded)? I asked about the phrasal verb 'treat of' here.

Answer 1

The answer to your question is twofold:

1) These are not really separate meanings in the sense of 'treat' and 'speak' being separate words. Just because two different 'senses' are listed in a dictionary, it does not mean that there are two separate meanings (like 'bank') in the minds of the speakers. You could easily see how 'speaking' about a certain subject in a certain way could be viewed as 'treating' it and the difference simply emerging from context.

2) There are many ways through which words acquire additional senses, sometimes even splitting into pure homonyms. They often involve things like metaphoric extension (as with logos), metonymy (as with 'foot' of the mountain or supposedly 'bank'), or some other figurative process. Frequently, the new senses become primary (as with 'awesome' or 'mistress'). Things are even more complicated when these developments happen across languages through poor learning or simply new contexts. New contexts also play a role in historical developments within a single language (as with 'sir').