Social semantics is more than just tags or vocabularies. It involves the users who contribute the tags, the perceptions of the world, and intentions that the tags are created for. Whilst social semantics is a valuable, massive data source for developing new knowledge systems or validating existing ones, there are also pitfalls and uncertainties. The epistemological analysis presented in this paper is an attempt to explain the differences and connections between social and controlled semantics from the perspective of knowledge theory. The epistemological connection between social and controlled semantics is particularly important: empirical knowledge can provide data source for testing the rational knowledge and rational knowledge can provide reliability and predictability. Such connection will have significant implications for future research on social and controlled semantics.