Here are some quotes to consider from "Defender of the Faith" by Philip Roth.

I had been fortunate enough to develop an infantryman’s heart, which, like his feet, at first aches and swells but finally grows horny enough for him to travel the weirdest paths without feeling a thing.

Nathan Marx, the narrator of the story, has been fighting in Europe, moving towards Germany at the end of World War II, when the story begins. He compares his heart to a soldier's feet that are hardened by marching. Just as a soldier's feet become calloused enough so that they don't feel anything, his heart has also grown toughened by seeing the misery and atrocity of war. He feels glad that he has hardened his heart against feeling.

He looked at me with those speckled eyes flashing, and then made a gesture with his hand. It was very slight—no more than a movement back and forth of the wrist—and yet it managed to exclude from our affairs everything else in the orderly room, to make the two of us the center of the world. It seemed, in fact, to exclude everything even about the two of us except our hearts.

Grossbart, who is in training under Marx, assumes that they will share a common understanding because they are both Jewish. His hand gesture is metaphorical of the world Grossbart creates that takes in Marx. He has a way of drawing Marx into his world and assuming that Marx will share his sensibilities. However, as Marx notes, this understanding leaves out his heart, which does not always side with Grossbart.

“Jewish personnel who want to attend services this evening are to fall out in front of the orderly room at 1900,” I said. Then, as an afterthought, I added, “By order of Captain Barrett.”

When Marx makes the announcement that Grossbart and the other Jewish trainees can attend Friday night services, he is careful to say that the non-Jewish captain, Barrett, made the order. Marx does not want to seem like he is giving advantages to other Jewish soldiers.

It had to reach past those days in the forests of Belgium, and past the dying I’d refused to weep over; past the nights in German farmhouses whose books we’d burned to warm us; past endless stretches when I had shut off all softness I might feel for my fellows, and had managed even to deny myself the posture of a conqueror—the swagger that I, as a Jew, might well have worn as my boots whacked against the rubble of Wesel, Münster, and Braunschweig.

Marx hears Grossbart singing, and the sound reaches down into his being and reminds him of growing up in the Bronx. This sound has to penetrate through him, as he has been hardened by what he saw in Germany. He has become hardened to any kind of romance or softness towards humans, and he has been almost emotionally anesthetized so that he does not even feel triumph as a Jewish conqueror in Germany.

There’s no limit to your anti-Semitism, is there?

At the end of the story, Grossbart accuses Marx of anti-Semitism because Marx refuses to grant Grossbart any favors and sends him to the Pacific. Grossbart feels that Marx should grant him favors as a fellow Jew, and he calls Marx anti-Semitic when Marx does not do so. This story questions whether Marx should grant Grossbart favors as a fellow Jew or be ruthlessly neutral about Grossbart.