The main theme of Primo Levi's memoir is implied in its title. Levi asks how men could treat other human beings as if they were not human, as the Nazis did the Jews and others whom they imprisoned and murdered during the Holocaust. The title also poses the inverted form of the same question: in other words, were the Nazis themselves men, were they actually human, when they perpetrated these atrocities?
During the journey to Auschwitz and his group's arrival there, the behavior of the SS men defies analysis. People are struck, clubbed for no reason, which leads Levi to ask, as most of us who have not experienced such violence would do, how one can strike another in the absence of anger? In his preface to the book, he states that whenever people believe that a stranger is automatically an "enemy," the path will lead to the existence of the Lager, the concentration camp. The tendency in people to do this, his says, is like an infection.
The SS refer to their prisoners as "pieces" when they do their roll call for the transport train. Once arrived in Auschwitz, the behavior of the guards, and the instructions given to the inmates, are surreal. The prisoners are told to strip, to place their clothes on the ground, and to guard their shoes so that the shoes "will not be stolen," as if there were any reason somebody would steal shoes in this setting. The prisoners are dying of thirst, but there is a faucet in front of them which is dripping water but has a sign over it saying "Drinking forbidden." They are shaved and then herded into a shower room but just left to stand there, while the officer has the interpreter tell them to be quiet because "this is not a rabbinical school." It is not simply that the prisoners are treated as not human, as objects, but that a gratuitous sadism marks the actions of their captors. The "selection," when it occurs (choosing who will be sent to the gas chamber and who will be kept alive), is a randomized thing, an indifferent process. Though if it had any logic about it, it would of course be just as barbarous. Levi nevertheless remained alive and was liberated in January of 1945, having been left at Auschwitz instead of evacuated as most of the prisoners were, before the approach of the Russian army.
Levi states that human beings are resistant to the concept of "the infinite," and therefore neither complete happiness nor complete unhappiness are possible in the human mind. To me it's not entirely clear how this concept is connected to his larger theme of inhuman cruelty, which, under the Nazis, seems actually to have the infinite scope one could not believe possible. At one point he states that he and the others, when all were without hope, were kept alive by the violence of the SS men towards them. It would have been easier to die, he implies, if their cruelty and sadism had been less severe. The reaction by the prisoners is the essence of what it means to be human. I would judge this point as one that both amplifies and paradoxically contradicts the theme at the center of his memoir. He asks "if this is a man?" as if the question of how the events he describes are possible in human life is unanswerable. Yet by staying alive, the prisoners have obviously proven their humanity, which should not have needed proof.