Defence lawyers for Mervyn Bell, who is accused of murdering a ten-month old baby in his care, say he cannot be found guilty of the crime because no doctor can say exactly how the infant died.
Mr Bell, 27, has pleaded not guilty to murdering and sexually assaulting the baby boy after taking him from his mother's house on March 19, 2013.
The infant had suffered horrific injuries from head to toe, including a broken arm and leg and numerous burns on his feet, legs and body. The boy also had severe injuries to his genitals and anus.
Mr Bell is accused of intending to kill the baby and inflicting injuries on the child over a 15-hour period before he sped into the Fortescue River Road House with the boy's lifeless body in his arms.
But Mr Bell claims the boy suffered the injuries after falling out of a car while it was moving around 60kmh, and was then burned later, when he was placed on hot rocks by the side of the river.
On what is set to be the final day of the judge-alone trial, defence lawyer John Myers said prosecutors had "fallen at the first hurdle", because they could not positively say how the infant died.
And Mr Myers also suggested that brain swelling evident in the child could be put down to hyperthermia, or overheating of the child, possibly suffered as temperatures soared to over 40 degrees C near Karratha.
Justice John McKechnie was told there was no real evidence to support the contention of a continuous, repeated assault on the child - and Mr Myers also questioned the motive.
"What was his motivation to injure the child ... to effectively torture the child? It is just not a realistic scenario," Mr Myers said.
Justice McKechnie has reserved his decision.