The thing I coudln't live without is my phone because of many reasons.  First thing that comes to me is, without a phone you can't call or text anybody.  I mean you can email someone but they would have to be on at the same time.  My second reason is if you don't have a phone and you need to call someone ASAP you can't unless you happen to find a pay phone.  These two reasons are why I think having a phone could possibly be the thing i couldn't live without.