This chapter is an application of some of Wittgenstein's thoughts on scientism to our knowledge of cases where we are threatened by potential devastation, and thus where clarity of thinking and action is most urgently needed - in particular, dangerous anthropogenic climate change. Scientism is perhaps the dominant ideology of our time. It is so dominant, at least in most intellectual circles, that it is hard to see it at all. In the public sphere, a lack of knowledge about the climate system is almost invariably taken to enjoin inaction. The Precautionary Principle (PP) is often criticised as being itself a recipe for inaction, when it is thought through to its logical conclusion. The chapter seeks, drawing on Wittgenstein and on philosophers who have learnt from him, to show how, far from being some recherché piece of philosophy exposed to damning objections, the PP should actually be seen as entirely defensible.