Canon 1573 of the 1983 Code of Canon Law confirms the validity of the evidentiary rule testis unus testis nullus in the canonical process. However, the requirement of two concurring witnesses is not absolute. The legislator provided for two exceptions. The first one is the deposition by a qualified witness concerning matters carried out ex officio. The second exception is when the circumstances of things or persons suggest otherwise. These are objective circumstances that reasonably make the judge to believe that the deposition of a single witness in the case can be accepted and that it can amount to full proof. The author takes a closer look at these two exceptions, with a special focus on their historical aspect, the jurisprudence, and the canonical case-law.