Heinrich von Kleist’s essay Über die allmähliche Verfertigung der Gedanken beim Reden, On the gradual making of thoughts while speaking, was probably written in 1805 or 1806 and published only in 1878. It opens with advice to a friend. If you want to know something and cannot find it by thinking alone, speak about it mit dem nächsten Bekannten, der dir aufstößt, with the next acquaintance you happen to meet. Kleist turns a French saying into l’idee vient en parlant, the idea comes while speaking.
Kleist places this thinking aloud in a conversation with a friend, not in a formal meeting.