I'm not sure this is the answer you are hoping for, but perhaps it would help to know that the sentence could just as easily have been:
The doctor talked of a long sea voyage… He thought it was a safe thing to say, for he knew the Shurins were poor and a long voyage was utterly impossible.
What was it? It was out of the question.
What was it? It was utterly impossible.
I hope this is helpful. If not, I'm sure someone else will have a better response.
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