Isadore Weisfelt, the father of Arthur Nielsen, was portrayed by Judd Hirsch.
A renowned professor of music, Isadore and Artie haven't spoken in decades due to an estrangement because of Isadore's belief that Artie had abandoned and forsaken music. Artie led everybody who knew him before to believe that he (Artie) had died, primarily for their own safety. Claudia Donovan, while attempting find Artie's childhood piano in Secret Santa, uncovered the fact that Isadore was very much alive. She lured Isadore to Leena's and, with Joshua's aid, she effected a reconciliation. Indeed, in their mutual griping at her high-handedness and inability to keep her nose out of other people's business, Claudia was spiritually as well as physically responsible for their getting back together.
In his one appearance to date, Isadore is as curmudgeonly as his son, but considerably more of a perfectionist, with a far lower threshold for other people's flaws, and a distinct tendency to speak his mind irrespective of the consequences. This last, however, is perhaps not one hundred percent true, since, in the midst of a tirade to Artie about the way Claudia manipulated him, he suddenly breaks off to check, "She's not yours, is she?", suggesting that he does have some degree of family feeling.
Isadore, although he has not been seen since Secret Santa, is fairly frequently mentioned, such as in The Greatest Gift, when Artie bemoans the fact that his father has invited himself along on what was intended to be a tête-à-tête with Vanessa. All's well that ends well, however, as Artie gets snowed in at the Warehouse, Vanessa can't make it either, and only Isadore and Hugo, who had also jumped on the bandwagon, are going to be left alone together in Sin City for the holidays.