In every speech you can say some things with certainty without lying.
But I think the point of the saying is in the other direction. If you are listening to a speech, the things that the speaker can say with certainty may not be the ones where you want certainty. And if you demand certainty on those things, you will find those who will give it to you. But the certainty itself is a lie - that's why the speaker can't (honestly) say those things with certainty.
What is the optimum political program for the United States? There are plenty of people willing tell you with (apparent) certainty what the answer is. The truth is that nobody knows with certainty, and so the answers that sound certain are lies. The actual program may be correct - may be - but the certainty itself is a lie.
This is often true in linguistics, and history, and politics, and economics. Don't demand certainty where there is none.
But I think the point of the saying is in the other direction. If you are listening to a speech, the things that the speaker can say with certainty may not be the ones where you want certainty. And if you demand certainty on those things, you will find those who will give it to you. But the certainty itself is a lie - that's why the speaker can't (honestly) say those things with certainty.
What is the optimum political program for the United States? There are plenty of people willing tell you with (apparent) certainty what the answer is. The truth is that nobody knows with certainty, and so the answers that sound certain are lies. The actual program may be correct - may be - but the certainty itself is a lie.
This is often true in linguistics, and history, and politics, and economics. Don't demand certainty where there is none.