Are fuzzy systems universal approximators?

This paper is a critical reflection on various results in literature claiming that fuzzy systems are universal approximators. For this purpose the most specific features of fuzzy systems are outlined and it is discussed to which extent they are incorporated in the formal definition of a fuzzy system in literature. It is argued that fuzzy systems can only be universal approximators in a rather reduced sense where some crucial features are neglected. The goal is to give an impulse to investigate more adequate mathematical concepts of a fuzzy system that also rake into account such features as transparency and linguistic interpretability.