Recent theorizing in entrepreneurship has proposed effectuation as a dominant decision model in entrepreneurial decision making. Through a verbal protocol study of 27 expert entrepreneurs who were asked to identify the market for a single new product, this paper establishes the existence of effectual reasoning in their cognitive processes and delineates the bounds between their use of causation and effectuation. In quantitative terms, over 63% of the subjects used effectuation more than 75% of the time.