 So a light clock will slow down, but does that mean a regular clock and time itself must also slow down? That's where the first posture that comes in. Let's say I want to train. I black out all the windows, no communication with the outside world. All I have is a light clock and a regular clock. So if the train is stationary, the regular clock and the light clock will stay synchronized with each other, as will all the biological functions in my body, you know, the rate at which neurons are firing in my head. Now let's say the train is moving with some velocity. We saw in the past section that the light clock will start to tick more slowly. Now the principle of relativity tells us that we cannot tell without looking outside the train how quickly we are going. So that must also mean that this clock I'm holding here must also slow down at exactly the same rate as that light clock. My biological functions should also slow down at the same rate because otherwise we'd have a way of telling, you know, how fast the train is going just by seeing how much more slowly this clock is ticking than the light clock. And this isn't some kind of weird assumption. Like fundamentally, the first postulate just comes from the idea that there is no absolute velocity. There is no one who can say I know I am still because the idea doesn't make sense.