It's true in physics – for every action, there's an equal and opposite reaction.
It's sometimes true in American politics, too.
So don't be surprised: The reaction and possible backlash to the last two days of Supreme Court rulings – especially the one legalizing gay marriage across the country – will likely be a significant issue in the Republican presidential race.
After all, nearly six-in-10 Republicans – 58 percent – said they opposed the Supreme Court legalizing gay marriage across the country, according to a national NBC News/Wall Street Journal poll released this week.
Already, we've seen different reactions from the GOP presidential contenders on the court’s gay-marriage decision.
There's anger and fight.
Mike Huckabee: "I will not acquiesce to an imperial court any more than our Founders acquiesced to an imperial British monarch. We must resist and reject judicial tyranny, not retreat."
Rick Santorum: "The Court is one of three co-equal branches of government and, just as they have in cases from Dred Scott to Plessy, the Court has an imperfect track record. The stakes are too high and the issue too important to simply cede the will of the people to five unaccountable justices."
Bobby Jindal: "Marriage between a man and a woman was established by God, and no earthly court can alter that."
Scott Walker: “I believe this Supreme Court decision is a grave mistake. Five unelected judges have taken it upon themselves to redefine the institution of marriage."
Ben Carson: "While I strongly disagree with the Supreme Court’s decision, their ruling is now the law of the land."
Lindsey Graham: "I am a proud defender of traditional marriage and believe the people of each state should have the right to determine their marriage laws. However, the Supreme Court has ruled that state bans on gay marriage are unconstitutional, and I will respect the Court’s decision."
There's disappointment but resignation.
Jeb Bush: "I believe the Supreme Court should have allowed the states to make this decision. I also believe that we should love our neighbor and respect others, including those making lifetime commitments. In a country as diverse as ours, good people who have opposing views should be able to live side by side."
And there’s the call for Republicans to appoint more conservative justices.
Marco Rubio: "This decision short-circuits the political process that has been underway on the state level for years. “While I disagree with this decision, we live in a republic and must abide by the law. As we look ahead, it must be a priority of the next president to nominate judges and justices committed to applying the Constitution as written and originally understood."
Maybe the biggest test will come a year from now – when the Republican Party is holding its political convention in Cleveland. Does its platform accept the Supreme Court's ruling legalizing gay marriage? Or does it keep its position that marriage should exist only between one man and one woman?