Middles across languages share a core of common semantic characteristics. Yet, they exhibit striking differences, as shown on a language-sample that includes Dutch, English, Hebrew, French, Italian, Polish, and Serbo-Croatian. The author argues that a single ARB-operation, inspired by the ARB-saturation of Chierchia for impersonals, can derived the shared semantic properties of middles across languages. The theoretical framework assumed in the study is Reinhart's Theta System. The analysis rests on a further development of the thematic feature-system. The author argues that when Middle Formation applies in the lexicon, it involves a feature-change, creating an ARB-role. When in applies in the syntax (LF), role manipulation is not available, and it involves the arbitrarization of a variable instead. The syntactic differences follow from a locus of the operation. Other proposed modification of the system facilitate the solutions to puzzles of theta realizations other than middles.