'Proto-idea' was a central concept in the thinking of the Polish microbiologist and philosopher of science Ludwik Fleck (1896-1961). Based on studies of the origin of the modern concept of syphilis, Fleck claimed that many established scientific facts are best understood as interpretations of prescientific, somewhat hazy 'proto-ideas' in the framework of a certain 'thought-style'. As an example, Fleck saw the modern knowledge of infection as an interpretation of the ancient proto-idea of diseases as caused by minute 'animalcules'. However, the epistemological aspects of the concept of proto-ideas have only been sparsely developed and discussed by Fleck and his critics. This paper attempts to bridge the gap. Firstly, I reconstruct the concept of proto-ideas in the context of Fleck's constructivist theory of knowledge. Secondly, I illustrate the relation between Fleck's concept of proto-ideas and his nominalist view on medical taxonomy. Finally, I discuss four philosophical problems implied by Fleck's concept of proto-ideas: (a) the problem of combining two conflicting perspectives on the history of science (b) the problem of accounting for the notion of 'continuity' within a nonrealist theory of knowledge (c) the problem of ascribing no truth-content to proto-ideas, and (d) the problem concerning the non-neutrality of the analyst's viewpoint.