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As remarked at the beginning of this section, the idea that explanation is connected in some way to unification is intuitively appealing. Nonetheless Kitcher's particular way of cashing out this connection seems problematic. Consider Kitcher's treatment of the flagpole example. This depends heavily on the contingent truth that some objects do not cast enough shadows to recover all of their dimensions. But it seems to be part not just of common sense, but of currently accepted physical theory that it would be inappropriate to appeal to facts about the shadows cast by objects to explain their dimensions even in a world in which Scientific software all objects cast enough shadows that all their dimensions could be recovered. It is unclear how Kitcher's account can recover this judgment.

The matter becomes clearer if we turn our attention to a variant example in which, unlike the shadow example, there are clearly just as many backwards derivations from effects to causes as there are derivations from causes to effects. Consider, following Barnes (1992), a time-symmetric theory like Newtonian mechanics, applied to a closed system like the solar system. Call derivations of the state of motion of planets at some future time t from information about their present positions (at time t0) , masses, and velocities, the forces incident on them at t0, and the laws of mechanics predictive. Now contrast such derivations mathlab with retrodictive derivations in which the present motions of the planets are derived from information about their future velocities and positions at t, the forces operative at t,and so on. It looks as though there will be just as many retrodictive derivations as predictive derivations, and each will require premises of exactly the same general sort — information about positions, velocities, masses etc. and the same laws. Thus the pattern or patterns instantiated by the retrodictive derivations look(s) exactly as unified as the pattern or patterns associated with the predictive derivations. However, we ordinarily think of the predictive derivations and not the retrodictive derivations as explanatory and the present state of the planets as the cause of their future state and not vice-versa. It is again far from obvious how considerations having to do with unification could generate such an explanatory asymmetry.

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