 These observations anticipate later criticisms of the DN model, which are commonly referred to as symmetry problems. In the case of the infamous tower-shadow example, most people have come to view only the derivation of the length of the shadow as properly explanatory, but not the derivation of the height of the tower (Bromberger 1966). Hempel (1965: 353n) himself admits that causal DN explanations seem “more natural and plausible” than non-causal ones. Woodward (2003) goes a step further by expounding a causal account of explanation using causal models.