Edward Targowski QC said Kimberley Hainey, a heroin addict and alcoholic with a history of mental health problems, could not be held responsible for the death of her son Declan.
The toddler was found dead in his litter-filled cot in the rubbish-strewn home he shared with his mother in Paisley in March 2010.
He had not been seen alive for months and experts who examined his body believe he may have lain dead for at least eight months.
Numerous friends, neighbours and relatives have told a Fatal Accident Inquiry (FAI) - which is trying to establish how Declan died - that they saw Kimberley alone on various occasions when she said that Declan was being looked after by her mum, Elizabeth Rodden.
Yesterday Mr Targowski told the inquiry that even if she had neglected Declan she could not be held responsible for him passing away.
He said: "I do dispute there was extensive neglect.
"I find it very difficult to see what is meant by extensive - whether it was every day that Declan was neglected or the odd day over several months.
"I don't accept there were up to 48 hours at a time that Declan was left alone.
"When is the first occasion of neglect closest to the death?
"In my submission that evidence does not exist."
The QC, who defended Hainey at her murder trial, also said that Hainey's position was that she had found Declan dead and had failed to cope with it.
Hainey, 39, was jailed for a minimum of 15 years in 2012 after being convicted of neglecting and murdering Declan.
But she was released last April on appeal after a three-judge panel quashed her conviction.
The inquiry, at Paisley Sheriff Court, was adjourned until today for other lawyers to give their submissions.