It is as fragile and elusive as a soap bubble, as protean as a Lava lamp. It is as hard to define as love or happiness, and even harder to trap and keep. This is your health we're talking about, the intangible that you probably think people like me can help you achieve and maintain. The operant fiction is that with diligent adherence to expert advice, pretty much anyone can sock away a nice little stash of health for the future.

In fact, though, health is the opposite of a commodity. It flits around like Tinkerbell, defying all the best intentions and predictions. No one can really articulate what the word means; no two people understand the concept in exactly the same way. And that includes you and your doctor.

Consider all the things that can be wrong with you even while you say you are healthy. You may have eyes that no longer focus on the printed page. You may be nursing a bum hip or a broken arm or leg; you may be missing an arm or a leg. You may have acne or a mouth full of cavities. Your toenails may be crumbling with fungus. You may have wrinkles and a tremor. Still, you rightfully call yourself healthy.

But then one day a few little viral particles crawl up your nose. A sniffle ensues, then congestion, fever, cough. Immediately, you are on the phone with your employer. "Not coming in today," you mumble. "Too sick."

Of course, you know you are not really sick, not sick in a way that negates your actual health. You are healthy in the midst of sickness, sick in the midst of health. Your bankable assets are secure.

It is a complicated journey, then, to get to the core of what makes health and what destroys it. The body ticks on through a lot of chaos. Even deep in our internal organs, where health is most often threatened, things can go badly wrong before we cross the river between healthy and sick. Women who get a diagnosis of breast cancer, for example, are usually quite healthy. It is the appendage that is sick. After they lose the appendage, they must take treatment that makes them very sick, so they can stay healthy. But, of course, some never quite make it back to that shore, for the terror of recurrent illness can itself negate health.

When epidemic infection came to town in the old days, it was usually clear who was sick and who was well. Yet in the midst of New York's typhoid outbreaks of the early 20th century, one of the healthy was Mary Mallon, "Typhoid Mary", the cook who carried the germ, infected dozens of others, yet never got sick. She died in quarantine, typhoid-free: the community's health will trump your own personal stash every time.

As we become healthier, the concept of health has ballooned to acquire a fourth dimension of time. It is not only enough to feel good today; we are not really healthy unless we feel good tomorrow, and the day after, until one morning we just don't wake up at all.