[OED:] [2.] a. {intransitive} To deal with some matter in speech or writing; to discourse. (In quot. 1517 transf. of pictorial representation.) Const. of, formerly also on, upon
What are the similarities and differences? Which definition of 'of' fits?
Is of a preposition or a particle?
I encountered this verb phrase below, but notice that the quoted clause with the bold originates from OED's entry on the suffix –logy.
-logy: word-forming element meaning "a speaking, discourse, treatise, doctrine, theory, science," from Greek -logia (often via French -logie or Medieval Latin -logia), from root of legein "to speak;" thus, "the character or deportment of one who speaks or treats of (a certain subject);" see lecture (n.).
Another example is the title of chapter one of Oliver Twist
Treats of the Place Where Oliver Twist Was Born and of the Circumstances Attending His Birth
Best Answer
Both speak and treat (meaning discuss) can be used either with objects (transitively) or without them (intransitively).
One can speak something--speak the truth. It is more common to use speak without an object.
Treat of (something) is relatively less common than treat (something).
Treat of is not a phrasal verb because its meaning can be readily understood in terms of adding denotations of its constituents: treat of = (essentially) talk + about.