The purposes of a border include protection. After the terrorist attacks of September 11, nobody can dispute our need for a border; we now have to discuss the exact nature and location of that border.

After the terrorist attacks of September 11, a number of Americans and Canadians speculated that some of the attackers might have entered the United States across the Canadian border. The evidence we have now indicates pretty clearly that even if some people connected with the attacks spent time in Canada, most of the perpetrators came from Europe, and spent most if not all the time before the attack in the United States.

This does not mean Canada and the United States have no reason to worry about security on the border. If terrorists from Europe or the Middle East to the United States encounter effective security when they try to come in directly, they will certainly try to reach their targets through Canada or Mexico. We know we need to improve border security; we now need to decide where and how.

A border security plan must accomplish at least two things: it must make both countries as secure as possible, and it must allow, as much as possible, for unimpeded trade. These goals conflict, and the Canadian and American government have put forward two proposals to resolve them.

The first, proposed by the American ambassador to Canada, envisions a "North American Perimeter", with visitors and immigrants subjected to a standard screening process on their entry to Canada or the United States, but allowed to move freely once within North America.

The Canadian government, in contrast, would prefer to have a modern, streamlined, and secure border between Canada and the United States. Partly, this reflects a prickly attitude about Canadian sovereignty, but it also takes a simple fact into account: a good border security plan will make it difficult for terrorists to move about North America. In the computer age, effective border security does not need to involve the long lines we saw at the border following September 11.

Instead, Canada and the United States could co-operate to build a system which would allow users to apply for a border crossing clearance on the internet, and at internet "kiosks" located at public places near the border. The clearance system would provide a crossing number the user could print up as a bar code, then scan at border crossing points before arriving at customs.

On leaving, each person would present the border crossing code again, allowing the security services on both sides of the border to record who had entered the country, when they left, and how often at when they visited. Nothing about this violates civil liberties; visitors to another country have an obligation to identify themselves.

I define a border as a line on the map which denotes a choice. In an insecure world, a border can also serve everyone's safety. People who wish to cross a border have to identify and explain themselves; a small matter to most people, but a serious check on terrorists. Canadians and Americans look out for one another, which makes the people of both countries safer; we can use the border between us to help us look out more effectively.

John Spragge is president of Dancing Cat Software of Ann Arbor Michigan. His Medicine Line columns appear from time to time in the Ethical Spectacle.