  Gassendi's compositionality thesis consists of the claim that the varied combinations of atoms give rise to all manner of physical, chemical, and biological features of the world and the phenomena they exhibit. Two elements of this thesis are noteworthy. First, atoms combine into particular molecular structures that correspond to particular macro-level features and phenomena. This pre-Daltonian view offers no suggestion that diverse collections of atoms are the source of molecular variety. Yet this molecularism is novel all the same for the broad suggestion that such micro-structures are intermediate-level building blocks of macro-structures, and for the particular proposal that the four Aristotelian basic elements be conceived of as molecular aggregates. Second, molecular structures en gros behave in the same ways as macro-level structures: they can be set in motion, pulled, elongated, compressed, and so on. In the case of molecular crystal structures, they can replicate, much as their macro-level counterparts can. On the other hand, there are significant dissimilarities among atoms and molecules: motion, density, and elasticity are all points of difference, which suggest the difficulty of a unified and internally consistent matter theory, relative to physical norms or laws. Yet Gassendi is confident that, at least in principle, the universe could have been created by God with any given composition of particles, as varying in size, shape, or any other dimension. This suggests that from that level of abstraction the laws governing motion (not contingencies for Gassendi) are intended to be scalar invariant. Even in cursory review, these tantalizing claims suggest significant and controversial consequences of stipulating that atoms have inherent weight and resulting tendencies to motion.