The letter set you are reading. It is stolen goods, improved.
Documented
The theft and the innovation are both documented
The Greek alphabet descends directly from the Phoenician consonantal script, transmitted around the 9th-8th century BC: the letter names, order, and early shapes match, and Herodotus's Cadmus account preserves the memory. The Greek modification was revolutionary: unused consonant signs were reassigned as vowels, aleph to alpha, he to epsilon, creating the first script that could write any word exactly as spoken, learnable in weeks rather than years.
Assessment: Fully documented in the epigraphic record. The borrowing and the vowel invention are textbook certainty, and the democratizing effect of vowel-writing is standard linguistic history.
Stories
Cadmus sowed the dragon's teeth
The myth the Greeks told about their own literacy: Cadmus, the Phoenician prince, founded Thebes by sowing dragon's teeth that sprouted into armed men. Later writers read it as allegory, the letters themselves are the teeth, and armed men rise wherever they are planted.
Assessment: The myth is documented, the allegorical reading is ancient, and both are the folk tier doing its job: the Greeks encoded the foreign origin of their letters inside their founding legends, in plain sight.
Interesting claims
Vowels made the mind audible
The strong claim, argued seriously by classicists in the 20th century: the voweled alphabet changed cognition itself, making abstract analysis possible by freezing speech into inspectable objects, and Greek philosophy is a downstream effect of the letter change. Literacy theory's alphabetic thesis: the technology rewired the thought.
Assessment: The alphabetic-cognition thesis was a major academic position, Havelock, Ong, Logan, and has been criticized as technological determinism: vowel-less readers philosophized too. What survives criticism is the narrower truth: for the first time, a spoken word and a written word were the same thing, and everything on this website is downstream of that.
Sources and further reading
Herodotus, Histories 5.58, the Gephyraean Phoenician letters
Wadi el-Hol proto-Sinaitic inscriptions, c. 1850 BCE (Darnell et al., Yale expedition publication, 2005)
Worth asking
The Phoenicians brought consonants; the Greeks added the breath. Was the alphabet a theft, a rescue, or the one improvement that made literacy belong to everyone?