According to commonsense, some collections of objects compose wholes, and others do not. However, philosophers have found serious difficulties with attempts to preserve this thesis, and especially with attempts to preserve the existence of just those composite objects recognized by commonsense. In this paper, I defend a classical solution to this problem: "it is the mind that maketh each thing to be one" (Berkeley, Siris, sect. 356). According to this view, which I call 'mereological idealism,' it is when a plurality is united in thought under a concept that a unified whole comes to exist. After explaining the view in more detail, I show how it escapes three standard arguments against commonsense views of composition.