Guard against people's hearts: people are unpredictable, and sometimes you have to be careful, careful, and don't trust people.

The abyss has a bottom, but only the human heart is unfathomable: the abyss has a bottom, and the depth of the human heart is more difficult to measure than the abyss. It means people are unpredictable. As the saying goes, the heart is separated from the belly, and I don't know inside and outside. That's what it means.

In short: teach people not to hurt others, and don't trust others, so keep an eye on them at all times to prevent being cheated.