Another controversy concerned what Robert Jan van Pelt termed Mayer's "well-meant but ill-considered reflection on the causes of death in Auschwitz". Mayer had concluded as follows: "certainly at Auschwitz, but probably overall, more Jews were killed by so-called 'natural' causes than by 'unnatural' ones." This conclusion removed the lingering restraints—- if any actually remained—- preventing David Irving from publicly announcing he did not believe the gas chambers at Nazi camps like Auschwitz existed. Guttenplan termed Mayer's musing on differences between 'natural' and 'unnatural' deaths, even if the terms were used in quotation marks, "indefensible".