Here’s what we can be fairly sure will happen in Monday’s presidential debate: Donald Trump will lie repeatedly and grotesquely, on a variety of subjects. Meanwhile, Hillary Clinton might say a couple of untrue things. Or she might not.

Here’s what we don’t know: Will the moderators step in when Mr. Trump delivers one of his well-known, often reiterated falsehoods? If he claims, yet again, to have opposed the Iraq war from the beginning — which he didn’t — will he be called on it? If he claims to have renounced birtherism years ago, will the moderators note that he was still at it just a few months ago? (In fact, he already seems to be walking back his admission last week that President Obama was indeed born in America.) If he says one more time that America is the world’s most highly taxed country — which it isn’t — will anyone other than Mrs. Clinton say that it isn’t? And will media coverage after the debate convey the asymmetry of what went down?

You might ask how I can be sure that one candidate will be so much more dishonest than the other. The answer is that at this point we have long track records for both Mr. Trump and Mrs. Clinton; thanks to nonpartisan fact-checking operations like PolitiFact, we can even quantify the difference.

PolitiFact has examined 258 Trump statements and 255 Clinton statements and classified them on a scale ranging from “True” to “Pants on Fire.” One might quibble with some of the judgments, but they’re overwhelmingly in the ballpark. And they show two candidates living in different moral universes when it comes to truth-telling. Mr. Trump had 48 Pants on Fire ratings, Mrs. Clinton just six; the G.O.P. nominee had 89 False ratings, the Democrat 27.