The purpose of this article is to make it clear that both Luhmann's system theory andmodern reason in philosophy treat the same problem as 'the world's complexity'. Luhmann argues that we need to differentiate a system from environment to reduce 'the world's complexity'. Only this differentiation can treat Husserlian horizon of human experiences. Luhmann thinks Husserlian intersubjectivity is useless because each person has one's ownintersubjectivity. Therefore Husserlian intersubjectivity does not give us the common picture of the world. In the post-Descartes philosophy, 'the world's complexity' is related to contingent knowledge. Modern science denied the existence of Aristotelian nous and Descartes invented self-consciousness instead. Since then there is a distinction between necessary truths and empirical contingent truths. Empirical concepts needs to be verified to be truths. This is thesame problem as` the world's complexity'.Rorty rejects Cartesian self-consciousness because it cannot determine which knowledgeis necessary. Language philosophy after Quine has demonstrated that there is no distinction between necessary knowledge and contingent knowledge. Only a self as a language network of beliefs and desires can treat this contingency of the world.