to somebody that's the natural direction.

对于某些人来说这才是惯常的方向
If you go to another neighboring country,

如果你们到了另一个相邻的国家
that's what they think is naturally x and y.

这就成了他们所惯用的 x 和 y 方向
There is no reason why x and y are nailed in absolute space.

x 轴和 y 轴在绝对空间中并不固定
It's very important that x and y are human constructs

很重要的是 x 轴和 y 轴是人为设定的
and we're not wedded to any of them.

且我们不能拘泥于它们
Quite often, it's natural to pick x and y in a certain way,

我们经常会很自然地选取 x 轴和 y 轴
because starting a projectile near the Earth,

因为如果在地表附近做抛物运动
it makes sense to pick the horizontal as x

我们通常以水平方向为 x 轴
and vertical as y.

竖直方向为 y 轴
Mathematically, you don't have to.

从数学上来讲没这个必要
Very important point that comes up is the following.

接下来要再提一个重点