In this paper, I will discuss the philosophical foundation of individualistic or radical constructivism and social constructivism with E. von Glasersfeld and K. J. Gergen as their leading scholars respectively. The main questions discussed are what their epistemological systems are, and what their philosophical foundation would be. I will argue that both theories still have to base on realism in terms of traditional epistemology. When applying ‘fit’ principle of radical constructivism, we can only apply personal experience in various situations for explanation and for creating our own meanings, but are unable to alter the fact which is open to everyone for different explanations. Based on Wittgenstein’s viewpoint, neither theory is able to rid of the realistic view of knowledge. Also, according to Wittgenstein’s later philosophy, it is not possible to create knowledge by personal language, since language cannot be the media of constructing knowledge and cannot create fact either.