Built in two distinct phases between 1897 and 1909, Glasgow School of Art is Charles Rennie Mackintosh's acknowledged masterpiece.

There is nothing quite like this building anywhere else. Yet uniqueness does not necessarily equate to greatness, so the question "just what makes this building so internationally important?" is a legitimate one.

Perhaps the first thing to consider is the context of the school's creation. Its client, Francis Newbery, the school's director, wanted a building to house what he was determined would be one of the great art schools of the world.

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Not for him any standard, fussy, high Victorian neoclassicism or neo-Moorish trendiness. Commissioning Mackintosh, a rising, innovative architect, ensured he would achieve his goal - and some!

Glasgow at the time was a city of great contrasts. Huge wealth co-existed alongside the poverty and overcrowding that beset the city's poor.

Yet there was a growing middle class, housed in the douce west end and in ever-expanding big-villa suburbia. And there was a keen appetite for education and the arts. Newbery chose his moment well.

The first phase of building followed previous themes which Mackintosh had explored in the Glasgow Herald building (now The Lighthouse). He wanted to create a synthesis of Arts and Crafts and Scots Baronial. However, it was in the second phase, with its crescendo in the magnificent Library, that his genius was truly off the leash.

The entrance, an embrace in curving stonework, gives into a low-ceilinged pillared hallway, drawing visitors towards a staircase within a timber enclosure and towards the intense natural light that drives down through it.

To the left and right, students and staff fan off towards their workplaces, the north-facing studios, whose spartan spaces have prompted some of the most ­internationally recognised creativity of the last 105 years.

The first real "wow" moment of the building is when the staircase brings you up towards the gallery above. At first-floor level the same, simple plan draws visitors forward to the Director's Suite, where the original furniture has welcomed innumerable staff, visiting artists, academics, architects and ­dignitaries down the decades, and off to studios and offices to left and right.

It is perhaps the users of the more private working spaces of the building who feel the greatest sense of affection for "the Mac".

They are the generations of students and tutors who have enjoyed its quirks and moments of magic - the gaps in the central spine up the linking stairs which run up the full height of the building at each end of the long corridors; the metal hangers, exquisitely detailed, which substitute for more traditional solid structure; and the famous "hen-run", everyone's private secret discovery.

However, it is at the end of the corridor which runs westward off the great gallery where what many would say is Mackintosh's greatest work of art in architecture is found.

The Library is an extraordinary space, its great oriel windows rising through three floors, and whose lighting, furnishings, fixtures, fittings, bookcases, gallery rails - every element - are the product of Charles Rennie's fertile imagination.

Fired by the work of the proto-Modernists in Europe and influences from the Orient, courtesy of JM Whistler and others, he created his dark wooded glade bathed in light from the west.

Each element in this complex composition was exquisitely crafted by artists of consummate skill under Toshie's watchful eye. In a building full of highlights and from a life full of creativity this was the crescendo, Mackintosh's greatest moment - a room which a world came to gasp at in quiet wonder.

It is the Library which has gone - and must be rebuilt. We owe it to generations past, and more especially to those yet to come.