Leonard Susskind, the inventor of string theory, is interviewed in New Scientist on the question "Is String Theory In Trouble." In the following interchange re. the anthropic principle Susskind uses an argument from ignorance.
The question is put to him: "Is it premature to invoke anthropic arguments - which assume that the conditions for life are extremely improbable - when we don't know how to define life?"
Susskind replies: "The logic of the anthropic principle requires the strong assumption that our kind of life is the only kind possible. Why should we presume that all life is like us - carbon-based, needs water, and so forth? How do we know that life cannot exist in radically different environments? If life could exist without galaxies, the argument that the cosmological constant seems improbably fine-tuned for life would lose all of its force. And we don't know that life of all kinds can't exist in a wide variety of circumstances, maybe in all circumstances. It a valid objection. But in my heart of hearts, I just don't believe that life could exist in the interior of a star, for instance, or in a black hole."
I have highlighted in bold his argument from ignorance. There is no evidence that non-carbon-based life does exist. Such non-existence threatens Susskind's multiverse theory and causes him to speculate. And notice how his final quote about his "heart of hearts" indicates his real doubt that non-carbon-based life does exist.
