The absence of von Neumann machines is one argument towards the uniqueness of intelligence to Earth. However, Anderson has conceived of different explanations for their absence.
"The nonexistence or extreme rarity of alien minds also has cosmological implications, ruling out certain models, which provides a separate test of the statement. I don't have room to go into that..." (p. 245)
How can the nonexistence or rarity of aliens have cosmological implications? I would have appreciated one sentence on that.
"From [these concepts] can spring some grand science fiction." (p. 247)
It has done, in Anderson's works. But the concepts have got me more interested in the real future. Can it be as herein described? We only ever have present fiction, not the real future. Or do we? We are now living in 2016, well after George Orwell's 1984 and Arthur C Clarke's 2001 and not long before James Blish's 2018. To our younger selves, we are living in the future.