The Conservative party chairman said that it was "impossible" for anyone to know "what really happens between a man and a woman" in a marriage, when asked about the public break-down of Charles' relationship.
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He declined to talk about the former couple in person, offering only a vague answer about the complexities of marriage during an appearance on BBC1's Andrew Marr Show to promote his new book, Brutal Simplicity of Thought.
"I can't end without asking you about your brother Charles, who has been through a very torrid time of it, and ask how he is and how you feel he has been treated by the media and public opinion," his host asked as the interview drew to a close.
"There are many aspects of human life, let's consider them – persecution mania, love, sex, marriage, work," Lord Saatchi replied. "These are all areas which produce great difficulties in people, and family and marriage is certainly one of the most complex areas."
Referring to his late wife, Josephine Hart, he added, "As Iris Murdoch said to Josephine Hart on many occasions, marriage is a private place.
"What Iris Murdoch was saying or suggesting was that it's impossible for anybody, even the best of friends of relatives of the people involved, cannot know what really happens between a man and a woman.
"When someone installs CCTV in people's bedrooms, which may be the next step of our surveillance state, then we’ll have the answer, but until then we don't know."
Charles, 70, and Nigella, 53, were granted a decree nisi at a hearing at London's High Court in July - the first legal step in ending their 10-year marriage.
Nigella applied to divorce the millionaire art collector on the grounds of his "continuing unreasonable behaviour".
Their relationship came to an end after pictures were published of Charles holding Nigella's throat at a top London restaurant.
Charles initially dismissed the incident as a "playful tiff" but later accepted a police caution for assault. He told the Mail on Sunday that the pictures gave a "wholly different and incorrect implication".
He added, "I feel that I have clearly been a disappointment to Nigella during the last year or so, and I am disappointed that she was advised to make no public comment to explain that I abhor violence of any kind against women, and have never abused her physically in any way."
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