Right. We attempted to put a self-sufficient colony in the American Southwest and still failed.
Surely it's fair to extrapolate our inability to succeed there into an inability to succeed in a far more harsh and remote locale.
Keep in mind that this isn't to say we could never succeed. Just that this is a very, very difficult problem that we don't even talk about seriously today, let alone study and pursue. Yet everyone just assumes it's somehow trivial and automatic once we decide to try.
