and processes. These include sensations and thoughts, and include both conscious and unconscious mental states and processes. World2 includes all animal as well as human mental experience. These mental states and processes only emerge as a product of biological activity by living organisms, and so only emerge subsequent to the emergence of living organisms within World1. World3: the realm of the 'products of thought' when considered as objects in their own right. These products emerge from human "World2" activity, but when considered as World3 objects in their own right they have rebound effects on human World2 thought processes. Through these rebound effects, World3 'objects' may - via World2-based human action on World1 - have an indirect but powerful effect on World1. In Popper's view, World3 'objects' encompass a very wide range of entities, from scientific theories to works of art, from laws to institutions. Popper makes two key claims regarding the role of World3 in the known universe. First, Popper argues that, despite the many continuities and correspondences between the human and animal World2, (1) only humans consider their mental products as objects in their own right in a World3 sense and (2) only humans have access to World3 objects. Second, World3 has no direct effect on World1 but only affects World1 as mediated by the human World2: for example, a theory of nuclear reactions will never of itself cause a nuclear reactor to be built, yet we can only understand the existence of a nuclear reactor by understanding it is in reality not the result of a purely World1 process but is the product of a complex interaction between particular World3 theories and human World2 mental activity, and then particular World1 actions by humans arising from this complex interaction. More on world3 Popper's world3 contains the products of thought. This includes abstract objects such as scientific theories, stories, myths and works of art. Popper says that his world3 has much in common with Plato’s theory of Forms or Ideas. But, world3 is not to be conceived as a Platonic realm, because unlike the Platonic world of forms, which is non changing and exists independently of human beings, Popper's world3 is created by human beings and is not fixed. It corresponds to the current state of our knowledge and culture. The interaction of world1 and world2 The theory of interaction between world1 and world2 is an alternative theory to Cartesian