Courts routinely resolve factual disputes as an adjunct to settling legal controversies, and such fact-finding frequently involves scientific and technical evidence. It is important to ask what intellectual resources judges bring to this task. Instead of assessing how much science judges know or understand, this essay focuses on the judge's role in articulating and reinforcing prevailing cultural attitudes toward science. Background judicial assumptions matter at three significant junctures. First, judges maintain the lay-expert boundary by deciding whether an issue demands expert testimony at all. Second, judges act as epistemological gatekeepers, by determining which expert claims and ways of reasoning are entitled to deference and which are not. Third, judges decide how to classify and categorize things of uncertain ontological status as a prelude to applying legal rules. Each kind of decision offers a window into judicial common sense, a relatively neglected topic in studies of law and science.