Why doesn't anyone talk about 18D? It's a cracker of a read. Every line packs a punch, every beat lands, every story arc is realised. It's not so much the sequel to 18C as its completion. That's because while section 18C of the Racial Discrimination Act tells you what you can't do – specifically insult, offend, humiliate or intimidate people on the basis of race – section 18D tells you what you can. It tells you all the things 18C doesn't mean; all the areas 18C doesn't cover. Read 18C on its own and you know nothing about it; it's only half the story. Stopping there and declaring your opposition is like going home at interval and whingeing you didn't like the show because it had a weak ending.

And yet, that's exactly what a parade of backbench senators did this week when they signed their anti-18C petition. Indeed, it's what the anti-18C crusaders have always done. "It shouldn't be illegal to offend" goes some version of the mantra. And that's a fair enough principle. But if that's meant to leave us with the impression that the minute you offend someone on racial grounds you've broken the law, then it's a lie.

Illustration: Simon Letch

Rejoice! Section 18D makes it plain that you're perfectly entitled to offend people in lots of circumstances. In fact, you're even allowed to humiliate and intimidate them. You're perfectly free to insult people if it's part of "discussion or debate" for a "genuine purpose in the public interest". You're free to offend – or humiliate for that matter – if you're making a "fair comment" on any "matter of public interest" based on a "genuine belief". The law requires only that when you do this, you're acting "reasonably and in good faith". Thanks to 18D, an offensive, insulting, racist argument that is nonetheless honest and genuine starts to look pretty safe.

You're not allowed to read 18C without 18D. And frankly, you shouldn't be able to talk about them separately either. Not if you're honest. And not if you want to be taken seriously. And yet, such is the senate that we're being asked now to take people seriously who don't seem to understand the law they're so convinced they oppose. Take David Leyonhjelm, who's launching a complaint against my Fairfax colleague Mark Kenny's "angry white men" column. His purpose is to demonstrate that column is illegal, which Leyonhjelm's himself accepts, it shouldn't be. Fine. But if Leyonhjelm loses the case, will he then admit that the law isn't nearly as restrictive on speech as he says it is, that he's fighting a straw man?