The Sages taught: Seven matters are concealed from people, and they are:
- The day of death;
- and the day of consolation from one’s concerns;
- the profundity of justice, ascertaining the truth in certain disputes;
- and a person also does not know what is in the heart of another;
- and a person does not know in what way he will earn a profit;
- and one does not know when the monarchy of the house of David will be restored to Israel;
- and when the wicked monarchy (which destroyed the Second Beit HaMikdash) will cease to exist.
Source: Pesachim, 54B.