Kent Roach teaches at the University of Toronto law faculty and worked with both the Arar and Air India commissions. Craig Forcese is a law professor teaching national security law at the University of Ottawa and a participant in the Canadian Network for Research on Terrorism, Security and Society.

The government wants to jail people who, by speaking, writing, recording, gesturing, or through other visible representations, knowingly advocate or promote the commission of terrorism offences in general, while aware of the possibility that the offences may be committed. We have completed and posted a 10,000 word legal analysis of this provision, and its constitutionality.

We have concerns. We do not accept as credible the claims that its scope is clear. We do not believe that it is confined to the objectives cited by the government's official backgrounder (penalizing someone who "instructs others to 'carry out attacks on Canada'"). Indeed, we think that this sort of statement is already criminal in most situations.

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We regard the proposed provision as potentially sweeping. We have serious doubts as to its constitutionality. Meanwhile, we have precisely no doubts that it is capable of chilling constitutionally protected speech.

We do not rehearse all our reasoning in this forum. We provide instead what we regard as a plausible hypothetical:

A newspaper columnist writing on foreign affairs is asked to present at a conference. It is the columnist's view that "we should provide resources to Ukrainian insurgencies who are targeting Russian oil infrastructure, in an effort to increase the political cost of Russian intervention in Ukraine". The columnist knows that her audience will include not just academics and Canadian government officials, but also support groups who may be sending money to those opposing Russian intervention.

Wisely, she decides to get legal advice. Her newspaper has no in-house experience with the new terrorism offence, and so (at great expense) it retains outside counsel. In a tightly packed five-page opinion letter, that lawyer reasons that if the columnist makes her statement, she will knowingly encourage a course of action that falls within the definition of a "terrorism offence in general".

This is because providing resources to a group, one of whose purposes is a "terrorist activity" is a terrorism offence. And causing substantial property damage or serious interference with an essential service or system for a political reason and in a way that endangers life, to compel a government to do something, is a "terrorist activity". This is so even if it takes place abroad.

The lawyer acknowledges uncertainty. "Terrorist activity" does not reach acts in an armed conflicts, done in accordance with the international laws of war. The lawyer consults with an international law expert, who opines that the expression "in accordance" with international law could exclude acts of violence by armed groups who lack what is known as "combatant's immunity" – that is, they are not lawful combatants. Few insurgencies meet the requirements of lawful combatants.

Fortified with this advice, the original lawyer advises the columnist that since she knows some of her audience may respond to her opinion by sending money to the insurgency, her acts may constitute the crime of promoting or advocating a terrorism offence. He notes that unlike equivalent "promotion" provisions in the hate crimes laws, there is no public interest defence that might apply to this situation.

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The lawyer advises the columnist to change her statement so that it reads: "Ukrainian insurgencies are targeting Russian oil infrastructure, in an effort to increase the political cost of Russian intervention in Ukraine. I take no position on whether this is a good thing".

An idea is changed, and an opinion hidden.

Government lawyers and politicians will accuse us of alarmism in deploying this hypothetical. They will say that no prosecutor would ever bring such a case. We would hope not. And likewise, we would also hope that no police officer would ever seek from a judge a highly intrusive anti-terror wiretaps in order to investigate this "terrorism offence".

And we would expect that if a prosecution were ever brought, a court would narrowly construe the offence's ambiguous language, or find some way to limit its reach. Or, as will probably happen years down the road, simply strike it down as unconstitutional.

But all this is beside the point. In a system built on the rule of law, we require law that limits poor judgment, not a law whose reasonable application depends on excellent judgment. This observation will recur in our assessment of many other provisions in Bill C-51. But here, it is especially true where an absolutely cardinal right is at issue – free speech.

In fact, it would almost be worse if no prosecution were ever brought. Until the law is narrowed or struck down by court judgment, everyone might feel the chill of this overly broad provision. Think about speech by people who are militant about foreign conflicts, Aboriginal self-determination or environmental matters.

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Some people will think that deterring speech with even the remotest tie to terrorism would be a good thing. If so, then you would have no sympathy for the columnist's predicament. We do, and we think this is a bad law.