"It must be sufficiently far off in time for the threat to last several years, long enough to force them into a real combination.
"It must obviously be a plausible danger and they must get to know of it in a plausible manner. Invasion from outside is the only threat so far suggested.
"It must be a limited threat. That is, it must appear to come from one well-defined group. The rest of the Universe should appear benevolent or neutral."
He just stops, rather as though there is something else to come; while the rest of us are waiting B sticks her oar in to the following effect.
"Yes, but look, suppose this goes wrong; it's all very well to make plans but suppose we get some of Kirsty's side-effects just the same, well what I mean is suppose it makes the mess worse instead of better we want some way we can sort of switch it off again.
"Look this is just an illustration, but suppose the Menace was pirates, if it went wrong we could have an Earth ship make official contact and they could just happen to say By the way have you seen anything of some pirates, Earth fleet wiped them up in this sector about six months ago.
"That would mean the whole crew conniving, so it won't do, but you see what I mean."
There is a bit of silence, then Aro says, "I think we should start fresh. We have had criticisms of Lizzie's suggestion, which was not perhaps wholly serious, and as Dilly says there is little of it left, except the idea of a threat of invasion. The idea of an alien intelligent race has objections and would be very difficult to fake. The invaders must be men from another planet. Another unknown one. But how do the people of Incognita come to know that they exist?"
More silence, then I hear my own voice speaking although it was my intention to keep quiet for once: it sounds kind of creaky and it says: "A ship. A crashed ship from Outside."
Whereupon another voice says, "Really! Am I expected to swallow this?"