The ongoing reform of investor-state dispute settlement (“ISDS”) underlines the pertinence of an old question that has received various and conflicting answers: Is investment arbitration a public or private method of dispute settlement? A key criticism leveled at investment treaty arbitration is that public interest disputes are decided by a system of private justice. This article critically reviews the dominant interpretations of investment treaty arbitration as public, private, or hybrid. It argues that the subjective nature of each interpretation means that none of them can be definitively adopted. Rather, the real arguments in favor of or against arbitration lie beyond the traditional debate. The article shows that investment arbitration displays important commonalities with international court systems, with its presumed unique features—including party autonomy—appearing a little less unique on closer inspection. Ultimately, a system is what states make it, irrespective of whether its particular features are described as public or private.