The fourth in a series of Gifford Lectures by Professor Michael Gazzaniga. Recorded 19 October, 2009 at the Playfair Library Hall, the University of Edinburgh. Audio version. So what does free will mean? It has become a catch-all term and means several things. In many ways the concept is fundamental to human thought and societal institutions. For example, our system of justice is built on the idea that we are all practical reasoners, working in a normal brain environment to produce coherent and ethical behaviours. We are held to be personally responsible for those decisions. Questioning the core concept, free will, necessitates rethinking many cherished notions of human institutions.