Coulomb's Law states that :
$$F_e = K_e \dfrac{q_1q_2}{d^2}$$ where $F_e$ means the electrostatic force between two charges of magnitude $q_1$ and $q_2$ and $d$ is the distance between them.
Scientist Charles Augustine de Coulomb derived it in the following manner:
He observed that $F_e$ was directly proportional to the product of the charge of the particles. So, $$F_e \propto q_1q_2$$ He also observed that $F_e$ is inversely proportional to the square of the distance between the particles (inverse-square law). So, $$F_e \propto \dfrac{1}{d^2}$$ Using these two proportions hence obtained, he concluded that :$$F_e \propto \dfrac{q_1q_2}{d^2}$$ which can be expressed in the form of an equation like this : $$F_e = K_e \dfrac{q_1q_2}{d^2}$$ where $K_e$ is the constant of proportionality and is commonly known as Coulomb's Constant or the Electric Constant.
Now, Coulomb observed two factors that affect the force of attraction between any two charged particles. But how did he conclude that there were only $2$ factors that affected the electrostatic force between particles? Could there not be a third factor which affects it too?
Or are we still not sure about the existence of any other factor?
Thanks!