Finally, Drs Aberegg and O’Brien assert that the real problem was an inadequate search for other possibilities, an explanation that could be used for any and every erroneous diagnosis that was based on a list of possibilities, no matter how long, if it did not include the ultimate answer. More importantly, however, many questions in medicine do not lend themselves to an easy search of textbooks or the medical literature. Aberegg et al5 propose teaching medical students to use Internet searches based on the chief complaint to ensure an adequate search has occurred. Following that strategy for “rising Paco2,” the resident’s description of the major problem in this case, I went through the first five pages of Internet hits and found no references that would give you the correct answer for this case, other than those that cite the article that is the topic of this discussion.1 I agree that we must slow down and use system II reasoning. But it is my contention, and the focus of the Interactive Physiology Grand Rounds series,6 that reasoning based on an analysis of a problem using basic principles of physiology and pathophysiology can lead one to an accurate diagnosis in these more complex cases.