While the exact phrasing "you learn by doing" isn't definitively attributed to a single person, the sentiment is associated with
Aristotle. He didn't say it in those exact words, but his philosophy strongly emphasized the importance of practical experience and embodied action ("praxis") in acquiring knowledge. Many interpretations of his work lead to the conclusion that "learning by doing" is a key principle in his teachings.
Therefore, while no one can definitively claim to be the originator of the precise phrase, Aristotle is often cited as the philosophical source of the idea.