Robin Thicke & Pharrell Williams -- If We Stole Songs ... So Did Beethoven!

If you're gonna throw the book at Robin Thicke and Pharrell Williams for stealing music ... you gotta take down none other than Ludwig van Beethoven too -- that's Robin and Pharrell's argument anyway.

The "Blurred Lines" duo just filed new court docs in their fight to prove the song is not a rip-off of Marvin Gaye's, "Got to Give It Up" -- pointing out several differences between notes in the songs ... and also backing up the bus over LvB.

In the docs, they admit 'Blurred' starts with repeated notes, just like Marvin's song does -- but also like Beethoven does in his famous 5th Symphony. All together now ... da-da-da-dum!

Their point -- musicians have been using repeating tones since the dawn of time ... or at least since 200 years ago when Beethoven was doing his thing.