An electron has a negative charge and a proton has a positive charge, we say. Meaning that both are opposites of each other in terms of their charges. Consider a hypothetical situation in which there are only electrons having originally negative charge in a certain universal space and we are unaware of this charge characteristic of the electron; then how would we ever know that it indeed has this kind of negative charge? Had it not been for the proton having a positive charge we could not have correctly classified them both, either ways. Therefore, which other subatomic particles of which these are composed of causes them to have charge characteristics and the subsequent electrostatic forces?