First, a physics lesson. For every action, Isaac Newton told us, there is an equal and opposite reaction.
So just as politicians everywhere must contend with the forces of anti-establishment populism and nativism that Donald Trump both harnessed and represents, so they are also faced with the furious reactions that the actions of the Trump administration are arousing.
They must deal with Trumpism but also with anti-Trumpism.
The outrage that exploded in the United States at the refugee ban announced last weekend by the Trump White House reverberated across the world.
It was especially vocal in the United Kingdom, and the British prime minister Theresa May was cautious and defensive in her answers to questions on the topic at Monday evening's press conference in Government Buildings.
The Taoiseach was more bullish. He strongly disagreed with President Trump, he said, and he was jolly well going to tell him so to his face (or words to that effect).
And on US immigration operations in Dublin and Shannon, where travellers can go through “pre-clearance” before they fly to the US, the Taoiseach was ordering “a complete review”.
And he said it in the tone of voice he uses when he really means it.
A day later, and following detailed briefings on the legality and utility of the pre-clearance arrangements, the Taoiseach was rather more subdued.
The Attorney General made it clear to Ministers at the Cabinet meeting that US immigration in Ireland was immune from Irish legal oversight, unanswerable to the Irish courts.
And anyway, the Government was very keen to keep it going, considering the benefits it brought. Even such sceptics of the Attorney's advice as Shane Ross (also happens to be transport Minister, of course) nodded their agreement.
As it happens – as we report in our lead story today – the AG's advice is disputed by various civil liberties organisations, but the Cabinet was having none of that.
The Taoiseach's previously trumpeted review would proceed, the Government spokesman said, though you would wonder why.
The episode demonstrated the extent to which politics in western countries like Ireland is being affected by the election of Trump.
Keeping on the right side of horrified European public opinion while maintaining some sort of relationship with the world’s most powerful country is going to be a very tricky balancing act.
Enda Kenny loves nothing so much in the calendar as the St Patrick's Day trip to the White House, and our leader endorses his decision to go.
But this year’s visit is shaping up to be a ceili in a minefield. And that is after a week and a half of the Trump presidency. Who knows what he will do before the middle of March?
More questions to the Taoiseach on all this are likely at Leaders’ Questions (at noon) and/or at Taoiseach’s Questions, which follow an hour later.
