What is being filtered out of our online platforms? What don't we get to see? That's a question that our guest worries about a lot. Niva Elkin-Koren is a Professor of Law at the University of Haifa, in Israel, and her research looks at the ways in which big tech companies (Google, Facebook, Twitter) are acting as "guardians of the public sphere", through content moderation. We ask her why she thinks that private filtering poses a fundamentally democratic problem, and we discuss her proposal for a "Public Artificial Intelligence."