The purpose of this paper is to understand and analyze Popper’s Theory of Three Worlds as a claim that “biological conditions,” such as the brain, are a necessary but not sufficient condition for consciousness. The Theory of Three Worlds is a pluralistic argument that posits a first world, the physical world, a second world, the mental world, and a third world, the world of objective knowledge. Popper calls for a third world for the following reasons. First, physical and biological conditions alone cannot guarantee the objectivity of scientific knowledge, and second, no scientific knowledge can be applied to the physical world without consciousness or perspective. As a philosopher of science, he marveled at the objectivity and rationality of scientific knowledge, but agreed with Hume’s problem of induction, which questioned its legitimacy. Popper believed that scientific knowledge is difficult to guarantee objectivity because it always presupposes a conscious being called an observer, and therefore argued that another world outside the physical world is needed to ensure the legitimacy of scientific knowledge. I agree with Popper and argue that we should reject scientism, at least the idea that scientific knowledge alone can explain everything about consciousness. This work emphasizes the need for humanistic studies such as philosophy in the study of consciousness.