Analogy is widely considered to be an important mechanism of scientific thinking and a source of creative insight in theory development. In this paper the implicit constraints that determine analogical soundness are considered. It first examines the constraints that govern analogical reasoning as it is predicted today. The scientific uses of analogy are then traced through three time periods and the styles of analogizing practice by scientists at different points in history are contrasted. This comparison suggests that the notion of analogical soundness has evolved over time. (Author/TW)