A.T. Robertson Commentary Acts 26:24

"And as he thus made his defense, Festus saith with a loud voice, Paul, thou art mad; thy much learning is turning thee mad." — Acts 26:24 (ASV)

As he thus made his defence (ταυτα αυτου απολογουμενου). Genitive absolute again with present middle participle. Paul was still speaking when Festus interrupted him in great excitement.

With a loud voice (μεγαλη τη φωνη). Associative instrumental case showing manner (Robertson, Grammar, p. 530) and the predicate use of the adjective, "with the voice loud" (elevated).

Thou art mad (μαινη). Old verb for raving. See also Joh 10:20; Acts 12:15; 1 Corinthians 14:23. The enthusiasm of Paul was too much for Festus and then he had spoken of visions and resurrection from the dead (verse 8). "Thou art going mad" (linear present), Festus means.

Thy much learning doth turn thee to madness (τα πολλα σε γραμματα εις μανιαν περιτρεπε). "Is turning thee round." Old verb περιτρεπω, but only here in N.T. Festus thought that Paul's "much learning" (="many letters," cf. John 7:15 of Jesus) of the Hebrew Scriptures to which he had referred was turning his head to madness (wheels in his head) and he was going mad right before them all. The old word μανια (our mania, frenzy, cf. maniac) occurs here only in N.T. Note unusual position of σε between πολλα and γραμματα (Robertson, Grammar, pp. 418, 420)