Fusion First[Note 1. I am grateful to Maegan Fairchild, John Hawthorne, John ...]

Logics of part/whole relations frequently take parthood or proper parthood as primitive, defining the remaining mereological properties and relations in terms of them. I argue from considerations involving Weak Supplementation for the conclusion that we should take fusion as our mereological primitive. I point out that the intuitions supporting Weak Supplementation also support a stronger principle, Weak Supplementation of Pluralities, and that the principle can only do the work demanded by our intuitions when formulated in terms of a notion of fusion that cannot be defined merely in terms of mereological properties and relations, logic, and a membership relation. So, insofar as we think any definition of fusion must be so restricted, we have motivation to take fusion as primitive; further, we have greater insight into the motivation for our supplementation principle and which version of that principle we ought to endorse.