Gerhard on why we can have pure doctrine

From a forthcoming volume on interpreting scripture:

§ 26. “The very thing that the prophets and apostles set forth to their hearers is the same thing that they put into the Scriptures by God’s will. It is not something different,” says Irenaeus (Adversus haer., bk. 3, ch. 1). Therefore just as those who heard the prophets and apostles could perceive from their words what the will of God was and what the mind and thinking of the Holy Spirit was, so we too can read the prophetic and apostolic Scriptures and from their books perceive what the will of God is and what is the mind and thinking of the Holy Spirit. The act of preaching and the act of writing are external accidents that do not change the essence of a thing. Just as oral speaking is the expression [ἀπεικόνισμα] of the mind’s thoughts, so writing is the expression the words from one’s mouth. The philosopher [Aristotle] discusses this (περὶ ἑρμην., ch. 1): “Spoken words are symbols of the perceptions of one’s consciousness, and written words are symbols of spoken words.”