Gaston Bachelard is one of the indespensable figures in the history of 20th-century ideas. The broad scope of his work has had a lasting impact in several fields - notable philosophy, architecture and literature. The Formation of the Scientific Mind is the work in which he first elaborated a theory of knowledge and its development which was to become a key to his thought as a whole - the notion of the epistemological obstacle - the unavoidable presence in the mind of a thinking individual of preconceived and misleading ideas derived from the very nature of language and culture. For Bachelard, the key to proper development of sicence (and indeed any field) was a sensitivity to the fact that knowledge advances against these epistemological obstacles - his demonstration draws extensively from the pre-history of science, alchemy, identifying instance after instance of preconceptions overcome at crucial junctures of development, and underlines an unavoidable conclusion: we cannot assume that these obstacles, even in the age of enlightened science, will ever be completely overcome.