Should we understand concupiscence as an actual sin or as original sin only? I started asking this question after watching the video featuring Drs. Timothy Keller and Kevin DeYoung wherein they talked their way through the AIC study report. While the AIC report and the accompanying video are helpful overall, we should not agree with comments made by Dr. Keller from minutes 27:52 to 29:05 of the video. During this exchange, Dr. Keller posits that concupiscent desires that are contrary to nature are no more heinous than concupiscent desires that are according to nature. To be specific, Dr. Keller argued that a man sexually desiring a woman not his wife and a man sexually desiring another man are both “equally illicit, equally wrong.”2 The question I asked myself immediately was, “How can that be? What about WSC Q.83? What about the qualitative difference between sins against nature and sins according to nature?” Dr. Keller’s comments thereafter brought the heart of the issue into sharper focus, “And we have to be very careful not to say, ‘Well, the desire for a man is unnatural desire, a woman is natural, so one of those is a more sinful desire than the other.’ This text is actually saying ‘no,’ that basically they’re both equally illicit, they’re both equally wrong, the capacity for sin is still wrong, it’s the original sin is what’s wrong with this and I think that’s very important, that we don’t create a little hierarchy inside.” Herein lies the issue—is concupiscence (i.e. unbidden sexual desire) a mere “capacity” or part of original sin and thereby exempted from degrees of heinousness, or is it an actual sin and can be more or less heinous? Though this might seem like hairsplitting, imprecision on this point leads to even greater problems further down the line as will be shown.