This paper deals with Ludwik Fleck's theory of thought styles and Michael Polanyi's theory of tacit knowledge. Though both concepts have been very influential for science studies in general, and both have been subject to numerous interpretations, their accounts have, somewhat surprisingly, hardly been comparatively analyzed. Both Fleck and Polanyi relied on the physiology and psychology of the senses in order to show that scientific knowledge follows less the path of logical principles than the path of accepting or rejecting specific conventions, where these may be psychologically or sociologically grounded. It is my aim to show that similarities and differences between Fleck and Polanyi are to be seen in the specific historical and political context in which they worked. Both authors, I shall argue, emphasized the relevance of perception in close connection to their respective understanding of science, freedom, and democracy.