“We all approach the id with analogies: we call it a chaos, a cauldron full of seething excitations...It is filled with energy reaching it from the instincts, but it has no organisation, produces no collective will, but only a striving to bring about the satisfaction of the instinctual needs to subject to the observance of the pleasure principle” (Freud, New Introductory Lectures on Psychoanalysis [1933]). 

“The ego represents what may be called reason and common sense, in contrast to the id, which contains the passions...in its relation to the id it is like a man on horseback, who has to hold in check the superior strength of the horse; with this difference, that the rider tries to do so with his own strength, while the ego uses borrowed forces” (Freud, The Ego and the Id [1923])