In the midst of such apparent confusion, there seem to be three elements which give unity to the novel:
The Role of Rastignac
A close observation of the novel shows that in the endless back-and-forth motion between the boardinghouse and the outside world, between Goriot and Vautrin, one character is inextricably involved: Eugène de Rastignac. He is the one who links the boardinghouse to the rest of Parisian society, who, because of his ambitions, becomes involved with Delphine, participates in Goriot's tragedy. He is also the character who appears constantly throughout novel, thereby giving it an element of unity.
The Role of Paternity
The omnipresent theme of paternity also gives a cohesive quality to the novel. Physical paternity in the title role, spiritual paternity in Vautrin's efforts to create an alter ego.
The Role of Society
But the real unifying force seems to be in the society depicted here and in its main working principle: money — the source of a preconditioning process which affects and explains the attitude of all the characters (except possibly Bianchon and Victorine). At the boardinghouse, we have seen the guests lodged according to their means. Mme. Vauquer's contempt for Goriot increases as the old man's wealth decreases. Mlle. Michonneau will betray Vautrin for money. Vautrin, himself, as a banker for the underworld, buys his influence. Delphine and Anastasie can only survive with money, which has become the yardstick for their love. And when he has no more money to give, Goriot withers and dies. Rastignac, himself, almost falls into Vautrin's trap because of money, and in the end, he has realized its vital importance.
This novel, therefore, finds its complete unity not as a novel of characters or as a detective story but rather in its social context. Placed by Balzac among his "Studies in Social Mores," it depicts a corrupt, ruthless society, but one in which one has to live, to which one has to adjust or experience the fate of Mme. de Beauséant, Vautrin, and Goriot, and in which many, like Anastasie, Delphine, and Rastignac, want to succeed at all costs.