每个时代的统治思想总是统治阶级的思想。
If you aren't in over your head, how do you know how tall you are?
The most important thing in communication is hearing what isn't said.
解释：处于安全环境时要考虑到可能出现的危险，考虑到危险就会有所准备，事先有了准备就可以避免祸患。