It is difficult, obviously. You can't say you can't tell someone what it is or you get into that situation we laugh about: “I can tell you what it is, but then I'm going to have to shoot you.”
There's a trust element, and the legislation, in section 39, makes it clear that Parliament is saying that if the Clerk of the Privy Council so certifies, that's the end of the matter and the court cannot examine it. There are other sections of the Canada Evidence Act dealing with national security and national defence, and the courts have developed a practice of actually looking at what this sensitive or injurious information is that the government is trying to keep out of the proceedings. This occurs typically in terrorist trials or something like that. The court satisfies itself that it really is something of national security and not just something that may embarrass the government. But there's no opportunity for the court to do that under section 39, and once it's certified by the Clerk of the Privy Council, it's beyond the reach of the court.
But the House of Commons is not a court of law and the House of Commons is not subject to section 39 of the Canada Evidence Act. The certification of something as a cabinet confidence does not, in my view, trump parliamentary privilege or the right of the House to receive information from the government and to hold the government to account.