Hawking, 73, suffers from motor neurone disease, and has argued that keeping someone alive against their own wishes is the "ultimate indignity".
In 2013, Hawking had said that terminally ill patients should have access to assisted suicide, as long as there are checks to prevent abuse.
In an interview for a new BBC programme, the scientist disclosed he suffered bouts of loneliness, because people can be afraid to talk to him or let him answer.
"I would consider assisted suicide only if I were in great pain or felt I had nothing more to contribute but was just a burden to those around me," he said.
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But, Hawking added: "I am damned if I'm going to die before I have unravelled more of the universe."
He is not in pain, he said, but suffers occasional discomfort because he cannot adjust his own position, 'The Telegraph' reported.
The neurodegenerative disease causes the nerve cells that control movement to degenerate.
Hawking has been wheelchair bound for decades and lost his ability to speak without an electronic voice in 1985.
When asked about what he misses about being able-bodied, he added: "I would like to be able to swim again. When my children were young, I missed not being able to play with them physically."
The programme to be broadcast on BBC One on June 15 will also feature interviews with Lucy, Hakwing's daughter, his youngest son Tim, and his Cambridge research students.