Scotland may leave before Quebec. In a recent issue of the Spectator, Allan Massie argues that Scotland's commitment to Britain has all but withered away in the wake of the Empire.

''It was after all the promise of admission to the commercial opportunities offered by empire,'' he writes, ''which had reconciled Scots to the loss of independence and to the Union with England.'' Participation in the ''imperial idea'' was the bond that had held Britain together. Now that it has been supplanted by the European idea, very little sense of attachment remains.

We are going to have to examine very quickly what it is that might hold Canada together in the future. This is not only a matter of Quebec, though the imminent failure of the Meech Lake Accord carries obvious hazards in this regard. The same centrifugal forces are present in every part of the country.

This has been obscured by the surge in pan-Canadian sentiment the constitutional crisis has provoked outside Quebec. But unless we grasp the need for a Canadian idea vaulting regional attachment, the moment will be lost. As for Quebec, it is not ''peace, order and good government'' that independantistes seek. It is inspiration. Might they find it in a Canadian empire?

It should not have escaped notice that while practically every other multinational federation is on the verge of dissolution, the U.S. stands as united as before. This is not because of the ''melting pot'' or unilingualism, both of which are largely myths. It is because Americans believe they owe a special duty to the world, a necessary greatness. The Soviets once believed this, too. An element of messianism is part of every successful nationalist movement. It cannot be otherwise with us. There is no doubt earlier generations of Canadians held that sense of mission in their hearts. Wilfrid Laurier's declaration that ''the 20th century belongs to Canada'' only reflected the enthusiasm of the day. Leacock heralded the ''inevitable greatness of Canada.''

But while the American sense of greatness remains, for generations of Canadian schoolchildren, Laurier's proud boast has come to seem a hollow curiosity. We have lost our sense of mission. Without it, we have lost our way, for like a shark, a nation must keep moving forward to survive. Every politician pays lip service to Canada's ''destiny,'' to ''this great country.'' Now we've got to mean it, and accept the sacrifices it requires.

The Canadian historian Frank Underhill defined a nation as ''a body of people who have done great things together in the past and who hope to do great things together in the future.'' Only when the Canadian idea is as compelling as the American, only when the national idea burns brighter in the mind than the region's or the province's, will the nation be secured.

Here is how we can begin. Let us have a moratorium of at least three years on constitutional change, to allow everyone to cool off. In the interim, the government should announce, immediately following the conclusion of the Uruguay Round of trade negotiations in December, that it intends to enter discussions with Japan and the European Community to reach a free trade agreement with both parties within two years, modeled on the Canada-U.S. deal.

Canada should slaughter whatever protectionist sacred cows are necessary to induce them to sign, with only one condition: that any agreement would apply only to Canada, and would not be extended to any part of Canada that tries to go it alone. Canada would stand, uniquely, at the crossroads of international trade, its coasts and borders commanding the three great markets of the world.

There's every prospect of this finding favor in Quebec, where support for free trade with the U.S. (and now Mexico) is strongest. We could present Quebec with a positive reason to remain a part of the Canadian federation: participation in the Canadian commercial empire of the 21st century.

That's just a start. Laurier's century is not out: In a rededication to greatness, we can rediscover our sense of common purpose. That means accepting the responsiblities of greatness, and demanding it from our leaders. We can ignore the mediocrity of Canadian political life no longer: for that is the style of a petty people, and we must be a great people. The historian W. L. Morton said we were called to greatness, but we are more condemned to it. We have no choice. We must be great or perish.