Imagine a scenario where you find a software bug. You go to another colleague. They perform the exact same reproduction steps you did. But the bug doesn't happen on their machine. What now?
Works on my machine
Your colleague may not believe you found a bug, or they may not be sure if you did. They may blame you for doing something you shouldn't have. They may insist that most users have a machine more like theirs than yours, and it doesn't matter if it doesn't work on your machine. They may think it's too much trouble to track down what's happening on your machine, and leave the burden to you to figure it out. They could have a fixed mindset, and think that you, your machine, the software you're running never change. (Read more about fixed vs. growth mindsets in this brilliant Brain Pickings article.)
Does not work on every machine
Instead, they could have started with belief. They could commend you for uncovering something they themselves could not. They could be curious about how your machine and software are different from what they have running, and look into how many other users this is affecting. They could pair with you to come up with ideas about how to stop the issue from happening. If they have more access to the underlying systems, they could look into the code and configuration settings. They could have a growth mindset, and think that your machine, the software you're running, and most of all you, can change.
Start with belief
Now, imagine a different scenario. Imagine someone describes being mistreated by the police. They were doing something that is fully within their rights, and the cops said they weren't allowed to.
I believe you.
Start with belief. Do not think that because you've had different interactions with the police, that the police must not treat people in the way you're hearing. Do not think that your individual circumstances, particularly the color of your skin, means that you'll be fine. (Think more about how racism is fascism applied to a particular category.) Take the time to shut up, listen, and discover how you can help. Believe that things can change. It starts with belief.