MAYBE IT'S BECAUSE John W. Hinckley Jr., the most famous patient on the grounds of St. Elizabeths Hospital, has spent so much time trying to get out for supervised excursions. Maybe it's because the District of Columbia clearly wants out of St. E's as well -- that is, wants to get out of the whole business of running the sprawling old facility and build a smaller, more up-to-date hospital to care for the city's mentally ill in this era of deinstitutionalization and Zoloft.

Whatever the reason, the security guard looks momentarily nonplused when, stopping my car at the stout iron gate of St. E's, I tell him that what I really want this bright fall morning is to get in.

"Are you meeting somebody?" he asks. When I tell him no, that I'm just hoping to have a look around, he hands over a visitor's pass and waves me toward Golden Raintree Drive, where to one side stands a lovely cluster of quinces, their leafy branches hiding hundreds of tiny autumnal fruit. Near the quince are a scattering of crape myrtles, still displaying their durable pink blossoms even now that summer is well past, as well as dogwoods with their bright red berries and fall-pinkening leaves.

Near those are just about any kind of tree or flower or bush you can imagine putting down roots in the mid-Atlantic region: white pines and blue Atlas cedars and luxuriant magnolias and a few trees dismembered by recent rainstorms and somewhere, one assumes, a golden raintree. As well as forsythia. Clematis. Cherries. In truth, I bet, the security guard does understand why I'm here. I'm here because the St. Elizabeths campus -- erected more than a century ago on a high expanse of ground that rises above Anacostia -- is one of the best places in this area to watch fall arrive.

At least, this is true if you're a voluntary visitor, I think as I cruise the shaded and winding streets of the more than 300-acre campus, trying to decide whether their names (Redwood, Sycamore and Oak Drives, for example), with their jarring air of Main Street normalcy, would have seemed taunting, somehow, to those who'd been plucked from Main Street and brought to live here instead. Back in the St. E's heyday, as many as 8,000 patients lived and suffered and even got well here, receiving what was considered the most progressive treatments. Among the best known was the poet Ezra Pound, who chose to spend 12 years in a room at St. E's rather than stand trial for making pro-fascist broadcasts during World War II.

These days, the facility houses just over 600 patients -- including Hinckley, who attempted to assassinate President Reagan but was found not guilty by reason of insanity -- as well as office space for some D.C. government employees. This morning, two of the latter (presumably) are out on the tennis courts enjoying a stiff-limbed but enthusiastic game. "Out!" and "ugh!" and "ack!" they shout, laughing and grumbling and frequently exiting to retrieve lobbed balls from the leaves and needles that have fallen from the massive trees spreading over the courts.

The mental hospital has long been an arboretum of sorts. The big trees were planted early in its history and new ones have have been added by a grounds crew that continues to seed and graft and nurture, not just for the dwindling patient population but, one imagines, for the pure love of horticulture, the challenge of growing a dwarf Kieffer pear. In the crumbling but still extensive greenhouse are baby azaleas and luscious hibiscus and big red geraniums -- mostly plants destined for ward decoration but some, like the odd cactus, simply kept here as pets. Outside that is a vegetable garden tended by patients and Anacostia community residents: Walking it in a deep state of melon envy, I stoop to inspect neat labels for "Indian Woman Yellow Dry Bush Bean" and "brocoli," which happens to be, I think, a very sensible misspelling.

A curious thing seems to have happened as patients have left St. E's and the facility has fallen into a bureaucratically indefensible but visually lovely state of cultivation and neglect. Namely, as you explore it you cannot help but notice instances not only of beauty but of what can only be described as the deepest sort of sanity. I'm talking about the tennis players, who had the good sense to leave their office on a day that called to them; about the bean-labelers and other careful garden-tenders; about the elderly ladies picking their way toward a picnic table in the grass; about visitors who come to savor the butterflies and the loudly calling birds. I'm talking about the men in work uniforms who, like me, have found their way out to the Point, a magnificent tree-hung promontory at the western edge of campus that affords an unparalleled view of the District and Virginia. Here -- on a high patch of ground overlooking the Anacostia and Potomac and the bridges spanning them and the traffic passing over them and the freeways branching off of them, the spreading treeless pavement of our own modern world -- one finds the perfect spot to contemplate not only the coming of fall but the difference, a constantly shifting one, between the madness above and the madness below.

Liza Mundy's e-mail address is mundyl@washpost.com.