This paper examines a debate over the certainty of scientific knowledge which has arisen in solar neutrino science. Material drawn from Interviews with participants is used to show the different perceptions of certainty available. It is argued that the craft element of science gives scientists confidence in their own results, but is also a source of uncertainty for scientists unfamiliar with the craft practices in use outside their own fields. The fundamental uncertainty of scientific knowledge encountered by scientists working at the research frontiers is discussed, and it is suggested that this type of uncertainty is unlikely to be revealed in interview material because scientists are aware of the possible public audience for their remarks. The implications of this for the analysis of public- science debates where scientific certainty is a contentious issue are briefly discussed.