How do I know what I think until I see what I say?

The title of this blog was one of the objectives we were supposed to think about, and the professor explained it in class today. However, I don't know if I agree with his explanation. Apparently, our thought process needs to see something written down before we truly know about it. Then exactly how did every pre-Sumerian (and the many cultures after the Sumerians that still did not have written language) know about their culture, and what message their stories contained?

Many nations used oral language to portray mythology, religion, and any other collection of tales. And who is to say that since these stories were not written down, the people had no idea what they were talking about and were just saying collective babble? Oral mythology is no less interesting or deep than written mythology. Take the story of Beowulf for example - the oldest European myth that we know of today. Beowulf was a mercenary in Geatland, modern-day Scandinavia, when a king contacted him to rid his kingdom of the troll Grendel that had been plaguing his people for years. What does Beowulf do? He takes his oversized sword and not only slays Grendel, but also chops off the trolls mother's head. He then probably returned to the king's hall, covered in blood and intestines, and feasted. Pretty awesome tale huh? Now that story has been retold orally for many hundreds of years before ever being written down.

To me, this all disproves that you need to see what you say before you know what you think. The entirety of oral tradition never wrote down anything, and the stories these people passed down have been preserved remarkably, and are some of the most talked-about and interesting myths of all time.