John Potter writes: Re. "Sydney Institute 'has diverse views'" (yesterday). Gerard Henderson makes the point that Clive Hamilton and Robert Manne have no qualifications in science or engineering. True, but neither do they have qualifications in woodworking, hairdressing or circus arts, which are as relevant to still-called global warming as engineering. Perhaps if Henderson found a hairdresser or bearded lady who shared his unscientific beliefs he might be cajoled into further relaxing his already malleable standards as to who may speak at his events.