They are the knicks and knacks that make up a life, the pieces of personal history that collect in attics and basements, closet tops and bottom drawers: a worn-out catcher's mitt, a first pair of toe shoes, a freshman essay on Chaucer, the brass knob from Grandma's front door.

What is trash to one person is a treasure to another. Some people surround themselves with empty wine bottles, old greeting cards, bits of string. Others feel strangled by too many possessions and delight in tossing things out.

But packrat or ascetic, hoarder or tosser, virtually everyone clings to some items "for no rational, definable reason," says Bethesda organizing consultant Sylvia Fogelman.

"People seem to keep objects that give them a kind of security. An executive holds onto an old college notebook, a widow keeps her late husband's favorite coat, a woman whose kids are now grown still saves all her old PTA files. In extreme cases, people will build a kind of fortress of possessions that they seem to feel protects their integrity or personality."

For some people, says Fogelman, "deciding what to save and what to throw out can be agony. Even those who are very bright and very well-organized still can't bring themselves to give up certain things from their past, despite the fact they've got a terrible space problem. I never insist that someone throw away something they're attached to. Our life is made up of memories, and you don't want to part with them all."

The items worth saving, she tells her clients--who pay her $20 to $30 per hour to create order out of chaos--"are the ones that have value to you." And what someone values, she says, "can tell a lot."

You are, she says, what you keep.

"Saving is a very individual thing," concurs estate-liquidator Anita Glick of A.M. Sales. "We've gone into houses where they've saved every TV dinner tray they ever ate off of, every box something came in, every greeting card, every empty lipstick tube they've ever used. Ultimately, I think they feel they're going to use it, but it's still piled up in the attic after they're gone.

"Then there are those people who are totally fastidious, with every snapshot pasted in a scrapbook, yesterday's newspaper in the trash and every article carefully labeled, sorted and put away."

Most people's saving habits, says her partner Marilyn Rudden, "fall somewhere in the middle. They hold onto things they want to pass onto their children, items that help them remember a special event and things that they feel might come in handy some day."

The "trash-or-save" decision can be particularly difficult for clients sorting through the possessions of someone close who has died. "People can develop incredibly strong emotional attachments to material things, notes Rudden. "And when they finally decide to sell something, they can't price it. A cookie sheet that to anyone else is just an old piece of tin, is to them the reminder of Mother's homemade oatmeal cookies."

Paper, says Virginia organizing consultant Barbara Hemphill, "is the No. 1 thing people tend to save.

"People save magazines and newspapers with articles about significant historic events like the death of a president or the hostages getting freed. When the Star folded they grabbed up last issues, some to hold as keepsakes and some in hopes of selling someday for a profit.

"Lots of people have clip files, particularly of 'how-to' articles and stories about medical discoveries. One of my clients had saved an article about a specialized new kind of jaw surgery, even though she didn't know anyone who had that problem.

"One of the most unique situations of clutter I've ever run across was that of a woman who had died, leaving her daughter 22 bureaus full of paper dating back to the early 1900s. There were grocery lists, receipts for bread at 5 cents a loaf, love letters, check stubs. It looked like she'd saved every piece of paper she'd ever gotten."

People save paper, Hemphill theorizes, "because they want to stay in control. It reassures them to have all this information at their finger-tips. The problem is, few people organize so they can fnd the piece of paper they need."

The second "most-saved" item, she says, "is a gift they don't like, but feel they ought to keep." Next, she says, is books, followed by loose change. "I once collected $400, scooped up from this dish or that dish."

Other popular saveables: "old glasses that don't even fit," "make-up that's never used but is too expensive to throw out," old toys, kitchen gadgets, shoes and items purchased when first married.

Older people raised during the Depression are more likely pack rats than younger people who grew up in an affluent "disposable society," says estate-liquidator Ann Lanigan. "The Depression mentality encourage saving, in case times got worse."

And in pre-synthetc days, items were made to last and were cared for meticulously.

"One very old lady who died had saved piles of exquisite lingerie dating back to the '20s. It was in beautiful condition and these young girls just fell on it screaming."

Also, says Lanigan, "older people may have lived in huge old houses with plenty of room to collect things. How much can you hold onto in a condominium or a townhouse?"

People who have moved a great deal tend to accumulate less than those who have remained in one place, claims New Yorker Stephanie Winston, author of Getting Organized. "When you move," she says, "you're forced to put a value on everthing."

The popular notion that "creative" people are greater clutter bugs than "straight business types," she says, "has absolutely not panned out in my experience." Nor, she contends, are women messier than men.

But Winston says the sexes do differ on one saving idiosyncracy: "Women tend to save quite a bit of clothing, hoping that the stlye will come back some day. Men get attached to one particlar item that they'll wear until it falls off."

People often save items that are "memorials of a great success, or a significant event," says Washington psychiatrist Sheila Gray. "That pair of track shoes you ran the marathon with may be a concrete reminder of an important moment in life."

Usually, she says, how much someone saves "is simply a matter of taste and style--not a pathology. Some people like empty rooms and some people like filled rooms.That's just the way they are."

Neatness or messiness can either be a "copying behavior" or a "neurotic behavior," maintains University of Maryland psychology professor Barry Smith.

"Copying behavior is effective. For some people, a high degree of organization and neatness helps them solve the problems of their life. For others, clutter or messiness may be the way they can effectively deal with the volume of things they need in their lives."

Someone who is compulsively neat or messy, however, is acting that way, he says," primarily to reduce feelings of guilt or anxiety. They have a terribly tense, free-floating fear that if they break their pattern something terrible will happen.

"Sometimes hoarding behavior will be the major symptom of what amounts to a neurosis. By accumulating more things, the person reduces anxiety, feels less guilty and more adequate."

To test whether behavior is "normal" or "neurotic," Smith suggests "making the person stop that behavior for a few weeks. A 'normal' person may not be very happy about it, but a neurotic person may have to be hospitalized."