Everybody knows what "smart" is, at least until they try to nail it down.

I'm thinking specifically about my friend N. The guy is no Einstein, if you know what I mean. His grades were poor. His SAT scores? Don't ask. Instead of college, he went right to work for an elderly couple who owned a store. They hit it off, and N inherited the store when the owners died. How lucky is that? Then he piled up a mountain of debt buying up real estate he couldn't afford, the idiot. But the market went through the roof, and now N is ... how shall I put it? Quite comfortable.

I have to turn the old adage on its head.

"If this guy is not smart, how come he's so rich?"

Here's the rub: Intelligence differs from person to person. My friend N, for example, didn't do well in school or on tests, but he's got something going for him, right? He's no dummy.

What sets one person's intelligence apart from another's is the type of complexity they're geared to handle. Michael Jordan, for example, could deal with basketball complexity in a game situation. Albert Einstein, meanwhile, could deal with complexity in mathematics.

So can we say there are multiple types of intelligence?

Intelligence is a slippery concept. It's not like strength, which is so nicely one-dimensional. If you can lift 100 pounds of lead, you can lift 100 pounds of wood. Weight is weight. If you can push hard, you can probably pull hard, hit hard, the works: Strength is strength.

But if you can solve a math problem, does that mean you can write a good sonnet? Design a dress? Beat the stock market? There's just no telling.

That's why nowadays educators, "educrats," and educational psychologists talk about multiple intelligences, a theory put forth by Harvard professor Howard Gardner in his 1983 book Frames of Mind.

According to Gardner, intelligence is not any single thing. It's not like gas in your tank that you can register on a dial and ring up as a single number.

Intelligence, Gardner says, is made up of at least seven, maybe eight, possibly nine, or even who-knows-how-many distinct abilities, working separately or in tandem. Each of us has our own distinct blend, our own individual recipe of intelligences--a dash of this, a cup of that, two or three ounces of the other. And some of us, you may have noticed, have more than one cup, more than one bucket even, of some ability or other.

And what are the intelligences? Here's Gardner's starting lineup, the original seven:

Lately, Gardner has added an eighth intelligence: naturalistic intelligence. This is the ability to recognize, sort, and find patterns in things like plants, animals, clouds, and rocks. Very useful if you happen to be a hunter-gatherer. (Some experts question the existence of naturalistic intelligence, but experts rarely agree on everything.)

Why do the abilities Gardner identified rate as separate intelligences? Why isn't humor in there? What about cooking? Did he pull these categories out of his hat?

Well, no, actually: Gardner had criteria for his pronouncements. He asked questions like: Can a test be devised to measure it? Is it plausible in terms of evolution?

Another question Gardner asked is: Can brain damage take this particular intelligence out of commission, while leaving other ways of functioning intact? Psychologist Oliver Sacks gives a great example of this one in his book The Man Who Mistook His Wife for a Hat (1985). Seems there was a music professor who lost his ability to judge or interpret anything he saw--he could easily mistake his wife for a hat. However, if he hummed or played particular pieces of music to go with his daily activities, he was okay and could function.

Another of Gardner's techniques is the simple observation that people can be really smart in one way without excelling much in other ways. Take Charles Darwin, for example. He was a so-so student, pegged for greatness by no one--until he went on a long voyage and saw countless strange plants and animals. Then suddenly--boom! He came up with the world-shaking theory of evolution by natural selection.

Frames of Mind and The Man Who Mistook His Wife for a Hat are available for purchase online.

It's safe to say the roots lie partly in heredity. It's safe to say that about anything we do. Safe but boring, because here we are now, and it's a little late to do anything about heredity, isn't it, even our children's?

So what else you got?

Well, nurture might be in there too.

Brain researchers have found that a newborn brain is quite plastic. Brain cells or neurons keep dividing for about nine months after birth. At that point growth stops, and you have to live with the trillion or so you've got because you'll never grow another. But that's no hardship, because everyone has enough brain cells to be the next Einstein.

The reason most of us aren't Einstein is that brain cells alone do not add up to intelligence. The connections among the cells, your "neural network," are more important in determining intelligence.

When you're born these pathways are in a primitive state. Every time new information comes in, it creates a new pathway through your brain as it travels from cell to cell, branching and splitting and converging. The next time a similar piece of information comes along, it is more likely to move along the established grooves. The more stimuli you get early on in life, the more pathways get hardwired into your brain, and this process goes on through adolescence.

The particular stimuli you get determine what type of neural pathways you build. Language is one obvious example. A newborn child has the potential to learn any language. A baby from Venezuela raised in Beijing will grow up speaking flawless Chinese.

Multiple intelligences might develop the same way. Does playing Mozart against a pregnant woman's belly make her child musically gifted? I don't know, but certain counterexamples are hard to argue with. If you raise a child in a room, isolated from all social contact, he or she probably won't grow up with much interpersonal smarts.

In short, some "intelligences" become more--or less--developed depending on the world you're growing up in. And, as the world changes, some intelligences may grow weaker while whole new ones emerge.

Case in point: Before the printing press was invented, there was a premium on memory. Back then people could routinely perform feats that seem incredible today. For example, the Indian epic Mahabharata, which is about 220,000 lines long--eight times longer than the Iliad and the Odyssey combined--was passed down orally for hundreds of years before it was written down. Who could memorize such a poem now? Hardly anyone. If you could, would it get you a job? Nah. The whole Mahabharata fits on one CD. At best, you might get a guest spot on David Letterman's "Stupid Human Tricks," right next to a guy who can lick his own collarbone.

Maybe that's why naturalistic intelligence is the last one Gardner defined. There's so little call for genius-level hunting and gathering anymore.

Is there a practical use for all this theorizing about different intelligences? Well, sure. It suggests that you can't excel at just anything; you'd better pick what you do. How many stories do we hear about kids who want to be lawyers, but their parents try to turn them into artists?

Okay, we don't hear many of those, but you know what I mean. Figure out what kind of intelligence you have, pursue a field where that's an asset.

Take heart. You might have an intelligence that hasn't been discovered yet. I, for example, can do something few others can match. I can sit at my desk with my keys in my pocket and lose them without ever getting up.

Okay, okay, I hear the muttering about "multiple stupidities." My point is really about my wife, who is my exact opposite. Anything I lose, she can find, even if she's never seen it before.

"Honey, I can't find my keys."

"Did you look in the freezer?"

Which intelligence is that?

Or let's go back to my friend N. Recently, he bought a used RV and set off on a road trip. He invited along a mechanic he happened to know. Since N was providing the ride, the mechanic agreed to pay for gas. They drove around for two wonderful months and had a great time. When they got home, N sold the RV, getting more for it than he paid in the first place. Let's summarize: A millionaire went on a two-month road trip in a gas hog and ended up richer than when he left.

I can't define it, but I know the guy has some kind of smarts. I only wish I knew what it was and how I could develop it.