Scottish Gallery of Modern Art: no rubbish in this collection.

There are (at least) two ways of collecting modern art - the backward way and the forward way. The first is to select, slowly and retrospectively, works representing artists and movements historically established. The other is to grasp at current trends, intuitively choosing works of pressing interest today, in the hope they will be of art-historical interest tomorrow.

Most galleries follow a little of both philosophies. The Scottish Gallery of Modern Art has, to date, mainly followed the first. In almost 25 years, it has quietly amassed (and far more by purchase than by gift or bequest) a carefully judged body of established 20th-century art.

Its continual growth has long called for a new home (since 1960 it has been ''temporarily'' housed in an appealing but inadequate 18th-century mansion in Edinburgh's lovely Botanics). Now, at last, it has moved into an early 19 th-century building of monumental proportions, a one-time school for destitute children not far from the west end of Princes Street in Edinburgh. Handsomely converted, the building was opened to a specially composed fanfare during the annual Edinburgh International Arts Festival in mid-August.

Two opening exhibitions marked the occasion. Downstairs was the most ambitious showing of items from the museum's permanent collection yet staged. Upstairs was a broad and bright array of 20th-century paintings (and photographs and sculpture) inspired by ''nature'' - an unusual approach to modern art, and revealing. In an arrangement somewhat like a Miltonic account of creation, there was in this exhibit, perhaps, a tendency to overpresent works of art perfectly capable of speaking for themselves.

When this initial installation comes to a close Oct. 14, it will then be possible to show fully - in a building five times the size of the previous one - the strengths and weaknesses of the permanent collection to date.

Already it is clear that this is no mean achievement. It began in 1959 with a mere nucleus of works by 20th-century Scottish artists (none living), one gift of a Klee, called ''Threatening Snowstorm,'' and one Kokoschka, called ''High Summer,'' which had been presented to Scotland by the Czech government in exile in 1942.

Today, largely as the result of the endeavors (and taste) of the gallery's Keeper, Douglas Hall, the collection boasts what he estimates as ''over 500 permanently framed works.''

Outside London, there is no comparable collection of 20th-century art in Great Britain. And London's Tate Gallery is, of course, entirely another matter - far larger, much broader in scope, older - in another league.

In spite of probably less than generous funding, and of being rather distant psychologically and geographically from the main centers of artistic activity, the ''GMA'' has sought out works by a respectable number of the generally agreed masters since 1900. We find here, to list a few, works (often only one, carefully chosen, per artist) by Picasso, Matisse, Leger, Arp, Mondrian, Giacometti, Ernst, Moore, Dubuffet, Derain, Rouault, Miro, Soutine, Jawlensky, Magritte, Kirchner, Nolde, Morandi. ... Some British artists - Ben Nicholson and Gaudier-Brzeska, for instance - are particularly well represented, and the collection of modern Scottish artists is unrivaled.

Possibly the collecting philosophy of wait-and-see is overcautious. It is undoubtedly an expensive approach. For instance a Lichtenstein, a Caro, and the gallery's first Pollock - a small work on paper - were not purchased until 1980: Wait for an artist to become a classic and he can't be cheap. And some of the larger and sadder gaps may be due to entering the field too late and too slowly: a Gris only on loan, no de Chirico, a dearth of Italian futurists, American Abstract Expressionists and Russian constructivists. The Tate has even lent a Rothko and a Gabo to help fill out the picture.

But one great advantage of thoughtful, delayed buying is evident: There is very little once-fashionable rubbish.