Rule of thumb for financial security

The following comes from Rules of Thumb:  A Life Manual -- Brilliant Guestimates, Shortcuts, and a Few Shots in the Dark by Tom Parker:

For a minimum level of financial security, your net worth (the cash value of all your assets) minus all your debts should equal one year's income. 

What is financial security to you?  How do you know when you're financially secure?

I'd love to hear your comments and discoveries.