Beethoven's Fifth Symphony

The fifth symphony was a famous piece of music composed by Ludwig van Beethoven.

The Twelfth Doctor explained the "bootstrap paradox" to the viewer, telling them to Google it. He uses an analogy of how he went back in time to meet his hero Beethoven, only to find out he would never write out the fifth symphony, so the Doctor copies it out based on his future knowledge, where Beethoven then publishes it. However this means the Doctor was inspired by Beethoven, who was inspired by the Doctor. He then leaves the viewer with the question "who wrote Beethoven's fifth?" before he takes out an electric guitar and plays the fifth symphony. (TV: Before the Flood)