According to the standard view, the function of the unlimited dunamis argument (Physics VIII.10, Metaphysics Λ.7 1073a5–11) is to introduce a new property of the first immovable mover, namely its lack of magnitude. The paper challenges this view and argues that the argument at issue serves to prove that the eternal motion of the first heavenly sphere is caused by an immovable mover rather than by a moved mover. Further, the paper shows that, at least in Phys. VIII, the unlimited (...) dunamis argument is the main argument for the immobility of the Prime Mover. (shrink)