Wisdom from the Early Stoics, Zeno of Citium 31

There are, as stated by Diogenes in his treatise on Language and by Chrysippus, five parts of speech: proper name, common noun, verb, conjunction, article. To these Antipater in his work On Words and their Meaning adds another part, the "mean."

A common noun or appellative is defined by Diogenes as part of a sentence signifying a common quality, e.g. man, horse; whereas a name is a part of speech expressing a quality peculiar to an individual, e.g. Diogenes, Socrates. 

A verb is, according to Diogenes, a part of speech signifying an isolated predicate, or, as others define it, an undeclined part of a sentence, signifying something that can be attached to one or more subjects, e.g. "I write," "I speak." 

A conjunction is an indeclinable part of speech, binding the various parts of a statement together; and an article is a declinable part of speech, distinguishing the genders and numbers of nouns, e.g. ὁ, ἡ, τό, οἱ, αἱ, τά.

—Diogenes Laërtius, 7.58