It was revealed in September that the 74-year-old comedian is battling with primary progressive aphasia - a type of dementia that affects the ability to talk - but his 'Monty Python' co-star thinks his condition is harder for his friend's loved ones to deal with than for Terry himself.
Speaking to the Radio Times magazine, Eric, 73, said: ''It doesn't seem to me that he's unhappy.
''I think it's harder for people around them than for the person. He hasn't forgotten who he is, yet. It hasn't got to that point. Terry's still here, he hasn't gone.''
The gang even helped Terry ''get through'' their live shows at London's O2 Arena in 2014.
Eric said: ''We said, 'Look, Terry, don't worry, we're going to get you through this.
''We're all in this together.''
Eric also claimed Terry - who also directed movies including 'Monty Python's Life of Brian' and 'Monty Python's Meaning of Life' - was ''having trouble stringing together sentences'' during the live shows, and is happy he's since gone public with the news of his condition.
He added: ''He was on stage there, but he was having trouble stringing sentences together and it was becoming noticeable - so I'm glad they came out with it and acknowledged his illness in public.''
Meanwhile, co-star Michael Palin, 73, previously said Terry's decline in health had been ''painful to watch''.
He wrote on Facebook: ''Terry J has been my close friend and workmate for over fifty years. The progress of his dementia has been painful to watch and the news announced yesterday that he has a type of aphasia which is gradually depriving him of the ability to speak is about the cruellest thing that could befall someone to whom words, ideas, arguments, jokes and stories were once the stuff of life.''
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