Defence QC Gary Allan said there had not been enough evidence at Malcolm Webster's trial to prove that the death of wife Claire, 32, had not been an accident.
Experts had been unable to say how Webster was supposed to have set fire to the car in which she was a passenger on a road near Kingoodie, Aberdeenshire, in May 1994.
"There is insufficient evidence to support the inference that Webster started the fire deliberately, with murderous intent," the lawyer argued.
Former nurse Webster, 54, was jailed for life in July 2011 and ordered to serve at least 30 years before parole.
Lord Eassie, sitting with Lady Clark of Calton and Lord Wheatley, are expected to hear two days of legal argument.