 While some of Avicenna’s physical-style arguments are quite complex and sophisticated, showing that the aggregation of bodies would simply be impossible on the Kalām atomists’ view (an example can be found in Section 2.3 of the entry on  Arabic and Islamic natural philosophy and natural science),  the following thought experiment is perhaps more intuitively obvious. Posit a sheet of conceptually indivisible atoms between yourself and the sun. Certainly the side facing the sun is distinct from the side facing you, for if the side that the sun is illuminating is the very same side upon which you are gazing, there is no sense in which the sheet of atoms is between you and the sun. Thus on the assumption that it is physically possible for this sheet to exist between you and the sun, all the atoms composing the sheet have a sun-side and a you-side, and these two sides are distinct. Purportedly, conceptually indivisible atoms have been divided, a contradiction. 