What is The Thinker Thinking About?

The sculpture known as The Thinker by Auguste Rodin is familiar to everyone. Even if you've never been to France and never studied art, you've seen memes of the statue, often on a toilet. But what inspired this muscle-bound intellectual and his contemplation? Who was he? And what's he thinking about, anyway? Although Rodin used live models, The Thinker is supposed to be a particular person that you know about.