I suspect they just mean that there's no particular part that seems impossible from where we sit now. There's no part that is prima facie impossible (eg a squa

I suspect they just mean that there's no particular part that seems impossible from where we sit now. There's no part that is prima facie impossible (eg a square circle). The problem of course is that we don't know enough to do it yet... so we also don't know enough to know that it's not doable. We may learn some reason that it's impossible along the way, but we haven't learned of any so far.