# Einstein Coefficients: What is the justification for absorption, spontaneous and stimulated emission reaching equilibrium?

In Einstein's derivation of the Einstein coefficients, he applied the condition for equilibrium on the two populations of atoms in processes of absorption, stimulated and spontaneous emission.

In doing that, he made compatible blackbody radiation with his mathematical description of the three processes, by setting the rate of change of one of the population zero to encode the state equilibrium in his physics.

My question is: what justifies that the two populations of atoms will reach equilibrium in the first place?