Popper's three worlds is a way of looking at reality, described by the British philosopher Karl Popper in a lecture given in August 1967. The concept involves three interacting worlds, called world1, world2 and world3. Worlds 1, 2 and 3 These three "worlds" are not proposed as isolated universes but rather are realms or levels within the known universe. Their numbering reflects their temporal order within the known universe and the fact the later realms emerged as products of developments within the preceding realms. Popper's theory of these three "worlds" is crucially a cosmological theory. As is consistent with the known universe as presently described by the natural sciences, Popper maintains that the known universe did not contain any World2 or World3 at its inception - at its inception there was only a "World1", a realm where everything consisted of physical states and processes. Moreover that "World1" was for a very long time devoid of any living matter and so was for a very long time a World1 lacking any biological level. The biological level is a level within World1 that emerged from its physical-chemical evolution over a vast tract of time, as a lifeless universe eventually gave rise to living organisms, such as those on earth. In a similar sense to this emergence of life within World1 itself, Popper's maintains that "World2" later emerged as a product of biological evolution, and that subsequently "World3" emerged as a product of evolution within the human "World2". This cosmological approach is strongly opposed to any form of reductionism that might suggest we can ultimately explain whatever comes later in the known universe from what came before - against this, Popper argues that we should see the universe as creative and indeterministic in that it has given rise to genuinely new levels or realms - like biological life, "World2" and "World3" - that were not there from the beginning and which are not 'reducible' to what was there from the beginning. The three worlds may be understood, in this evolutionary and cosmological sense, as containing three categories of entity: World1: the realm of states and processes as studied by the natural sciences. These include the states and processes that we seek to explain by physics and by chemistry, and also those states and processes that subsequently emerge with life and which we seek to explain by biology. World2: the realm of mental states