This quote was stated by Baron de Montesquieu, a french philosopher. Montesquieu strongly opposed the aristocracy prevalent in France and condemned the corrupt practices conducted by them.
His contributions to political theory are still revered and studied by many scholars even today. He advocated natural human rights and was an eminent philosopher of the Enlightenment Period.
Montesquieu argued that when legislative and executive powers are given to the same person, he/she may become corrupt and tyrannical.
In this way, the common people would lose their basic rights and freedom because higher individuals with too much power become despotic and dictatorial.
