 According to theory reduction, theory TA reduces TB (e.g., Atomic physics reduces Biology) if TA logically entails TB. Given a deductive-nomological account of  explanation  (Hempel and Oppenheim 1965 [1948]), theory reduction as deduction from theoretical principles is an instance of explanation. In particular, TA logically entails and explains the laws of TB. Compare this with the logically weaker notion of reduction put forward by Kemeny and Oppenheim (1956), which includes instances of theory replacement. The Kemeny-Oppenheim model concerns the situation where the observable predictions made by TA include (at least) all observable predictions of TB. In this case TA can explain everything that TB explains, but TA need not encompass the theoretical principles of TB (as in genuine theory reduction), which allows for the possibility that TB contains inadequate theoretical notions and is replaced by TA.