There are certain existential questions that humans have wrestled with since the dawn of time: “What is the meaning of life?” “Which came first: the chicken or the egg?” Perhaps even “Does the refrigerator light stay on when I close the door?”

Recently, New York City Mayor Michael Bloomberg tried to answer another such question. “If government’s purpose isn’t to improve the health and longevity of its citizens, I don’t know what its purpose is,” he declared on CBS. Hence, his move to ban large sodas in his fair city.

If the mayor wants to educate himself about why government truly exists, he’d do well to review the Declaration of Independence. Almost 236 years after it was signed, it still provides the best explanation ever written of the proper role of government.

“[M]en establish governments to secure their pre-existing natural rights. Where there is no government, rights are easily threatened by others, since the coercive power of the state does not function as a deterrent,” explains a Questions & Answers page on the American founding from The Heritage Foundation. “The purpose of government is therefore to create the conditions that allow each individual to freely exercise his rights.”

No mention of soda. Or transfats or even salt, two other things Bloomberg has moved to regulate. That’s because it’s not government’s job—at any level, federal state, or local—to make decisions about what, how, or how much we eat. A government that tries to do these things becomes a nanny state, acting like a parent.

Except, unlike an actual parent, it doesn’t want you to grow up and make your own decisions; it wants to order you around as if you couldn’t possibly be trusted to make your own decisions. Even about something as fundamental as what to eat.

The bottom line is that, in order for government to play its proper role, it must be properly limited. Governments that tell us what to eat or how to light our homes (“We are taking away a choice that continues to let people waste their own money,” Energy Secretary Steven Chu said last year about the government’s ban on incandescent lights) are too large and intrusive to do what they’re supposed to do: protect our natural rights.

Bloomberg ought to grab a copy of the Declaration. It’s the first place to turn when you have any questions about the reason for government.