The educational literature recognizes that people go through a number of stages in their intellectual development. During the first stage, called "received knowledge" or "dualism", people expect knowledge to be handed to them by authority figures (thus "received") and think in terms of black and white (thus "dualism"). Our experience indicates that many computer science students are at this first stage of learning. To help students move beyond this stage, we describe a system and strategy, the PL-Detective, to be used in a Concepts of Programming Languages course. Assignments using this system directly confront students with the notion that they can create knowledge via interactions with the PL-Detective and that discussion with students (rather than asking the instructor) is an effective way of learning how to reason. We present experimental results that show that the PL-Detective is effective in helping students move beyond the stage of received knowledge.