 The First Day also contains Galileo’s account of the acceleration of falling bodies, and the argument that they fall equally fast in a vacuum, whatever their weight. This discussion contains the famous thought experiment refuting the Aristotelian theory of fall, according to which the speed of a body’s fall is proportional to its weight. In this “short and conclusive” argument, Galileo supposes that two bodies, one heavier than the other, are suddenly conjoined in the midst of falling. On the one hand, if Aristotle is correct, the faster fall of the heavier body will be retarded by the slower motion of the lighter body, so that the conjoined body will fall slower than the original heavy body. And yet, the conjoined body is heavier than either original body, so it should also fall faster. Hence, there is a contradiction in the Aristotelian position (Gendler 1998; Palmieri 2005; Brown and Fehige 2019).