Imagine you are trying to convince your friend to eat healthy food. You tell them all the good things about healthy food and how important it is to eat healthy. The first thing you say has a big impact on what your friend thinks about your message.
The law of primacy in persuasion, simply put, means that the first thing you hear about a message has a strong influence on what you believe. If you hear something first, you are more likely to remember it and give it more weight in your opinion.
This is why people who want to persuade others often try to make a good first impression, by presenting their strongest argument first. They know that if they can capture your attention and convince you with their first point, you are more likely to be persuaded by the rest of their message.
So, when you are trying to persuade someone, remember to put your strongest argument first so that it has the best chance of convincing them. And when someone is trying to persuade you, remember that the first thing they say can have a big impact on what you think about their message.