What is heresy in an age in which there are many competing truths, but no certain one? What are the roles of heresy in the modern contexts of religiosity and secularism? What does heresy have to do with loyalty to tradition? And what is the meaning of a culture without the possibility of heresy? The lecture series titled "Heretical Thoughts", which was held at the Van Leer Jerusalem Institute in 2019, sought to reexamine the meanings of “heresy” as they developed in religious thought, and especially the way the notion functions today in a secularized world. The speakers discussed the fundamental truths of our society, reexamined the binary division between heretics and believers, asked who the heretics of our day are, and wondered what their cultural role is in a secularized world.