Why is the value of RSSI negative?

Hello Planck,
Many people wonder why the RSSI is negative. In other words, why the received wireless signal is negative. Most radio signals are at the mW level. Therefore, polarizing them and converting them into dBm does not mean that the radio signals are negative. For example, 1 mW is 0 dBm, and less than 1 mW is a negative number.
Definition of Signal Strength: RSS=10logP
[Example 1] If the transmit power P is 1 mW, the value is 0 dBm after the conversion.
[Example 2] For the 40 W power, the converted value in the unit of dBm is as follows:
10lg（40W/1mw)=10lg（40000）=10lg4+10lg10+10lg1000=46dBm。
I hope it helps!