 Causes that are essentially ordered are simultaneous, such that the prior stages are a condition for the effect’s ongoing existence, as when waves move a ship, the wind moves the waves, and the wind is moved by elemental forces (QPhys 7.25 {235}; Incoherence I.1 {59}). In such a series there must be a first cause, because an endless such series would be actually infinite all at once, which Ibn Rushd regards as impossible (Incoherence I.4 {275}). The series could, moreover, never reach its end, since causal agency cannot pass through an infinitely long series in a finite time (LongPhys VII.6; QPhys 7.25 {235}). This First Cause cannot itself be something in motion, given the Aristotelian dictum that everything that is in motion must be moved by another (LongPhys VII.1; QPhys 7.30 {236}). In principle, Ibn Rushd allows that there could be many unmoved movers (QPhys 7.35 {238}), but at a minimum there must be one such thing that is immovable, and eternally so, because otherwise some still prior mover would be required to move the supposedly first mover, and this would lead again to an essentially ordered infinity of movers (EpiMeta 4.139 {126}). (For the intricate details see Twetten 2007.)