I really believe that brotherhood is what makes a man human.
Moses says this to his friend, Lucas Asphalter, in the seventh section of the novel. Moses has just flown into Chicago to visit his friend and confront his ex-wife and is staying with Lucas for the night. They have a philosophical discussion of sorts, and Moses, toward the end of the conversation, expresses this thought. This is important because Moses has moments of pure joy in which he experiences being at one with his fellow human beings. For example, he realizes the "universal communion" of human beings when he is walking in the subway station and thinks about all the hands that have touched the railing he is touching.
The irony is that Moses comes to this conclusion about connections with society within the solitude of his own mind. Nevertheless, there are moments that he shares with his brother, Lucas, and others in which he experiences love—a love he calls "potato love." At first he questions this common love and at times he rages against his sensitivity but, nevertheless, by the end he begins to understand that it is essential. This quote is also related to the above quote (the first in the section) because it is important to understand that Moses must come to understand himself before he can communicate with others.