Black, Wilson, and Yao (this issue) commendably attempt to put descriptive theory at the center of pedagogy, assessment, and curriculum. The thrust of their article is that only through theories of learning will student progression be properly understood. Casting a critical eye over the faddish distinction between "formative" and "summative" assessment, they note there is no real substantive difference between them. Recognizing the need for something more solid than the mere labels of "formative" and "summative," Black et al. proposed that both uses of assessment be "based on a common "road map" that can serve as a "backbone" for a learning progression, and that both have been built to be consistent and supportive of that road map." That is, explicit theory as to what is being learned (curriculum) and how (pedagogy) should drive assessment. In particular, the curriculum "reflects and provides a strong model of progression in learning. This "road map" may then inform both pedagogy and the assessments." Black et al. present the atomic theory of matter as an example road map. From a thorough review of students' misconceptions of changes in the state of matter, they deduce yet another road map. Both "road maps" are descriptive theories of how students progress in their understanding of the atomic theory of matter. The striking thing about them is that they imply that student progression is not measurable. In this article, the author commends Black et al. for developing a descriptive theory of learning and attempting to use it as the basis for assessment. This kind of thinking should be encouraged in education and psychometrics generally. Unfortunately, the author contends that despite their critical attitudes, they wavered in the face of the quantitative imperative and attempted the scientifically impossible: the measurement of a partial order.