Montesquieu explained his theory in this word: “in every government there are three sorts of power: Legislative, Executive and judicial. The liberty of individual required that neither all these powers nor any two of them should be placed in the hands of one men or one body of men.
(1). When the legislative and executive powers are united in the same person or body , there can be no liberty, because apprehensions may arise that the king, who is also law maker, might make and enforce the laws in a tyrannical manner.
(2). If the judicial power is joined with the legislative , the life and liberty of the subject will be exposed to arbitrary control, for the judge would then be the legislator.
(3). Where the judicial power joined to the executive power, the judge might behave violence and oppression.
(4). There would be an end of everything if the same man or the same body, whether of the nobles or of the people, were to exercise those three powers, that of enacting laws, that of enforcing them and of trying the cases of individuals