The basic ideas of Existentialism can be seen in the The Diary of a Madman. The protagonist and his relationship to the world around him is where the basic precepts of Existentialism lie. This relationship highlights the condition of being alone and the tension between he and the external world.
Part of where one sees the presence of Existentialism in the The Diary of a Madmen is in the lack of a divine force. In the diary entries, it is evident that there is no totalizing or unifying force that constructs individual identity. Human behavior in the absence of the divine consists of perceived cannibalism and a lack of certainty. This is seen in how the narrator struggles to understand the world and his place in it:
They [people around the narrator] seem to have secrets which I cannot guess, and once they are angry they will call anyone a bad character. I remember when my elder brother taught me to write compositions, no matter how good a man was, if I produced arguments to the contrary he would mark that passage to show his approval; while if I excused evil-doers, he would say: "Good for you, that shows originality." How can I possibly guess their secret thoughts—especially when they are ready to eat people?
There is a lack of understanding regarding social interaction and about human beings, in general. There is no totality and little in way of absolutist structure to guide human actions. There is paranoia, insecurity, and doubt within human consciousness. This reflects a universe where the individual is alone. This shows how the narrator is unable to turn to anyone for support and feels completely cut off from others. At the same time, the questioning of the individual and the world around him is a reflection of the Existentialist tendency to experience a disconnect with the world around the individual.
Existentialism focused on the struggle of human existence. The only certainty in the world for the Existentialist is the struggle of being. Struggle is what defines individual consciousness. This can be seen in the madman's diary, as well:
But just because I am brave they are the more eager to eat me, in order to acquire some of my courage. The old man went out of the gate, but before he had gone far he said to my brother in a low voice, "To be eaten at once!" And my brother nodded. So you are in it too! This stupendous discovery, although it came as a shock, is yet no more than I had expected: the accomplice in eating me is my elder brother!
The eater of human flesh is my elder brother!
The narrator's struggle in being in the world is one that revolves around cannibalism. Who is a cannibal, how they are focusing on him, and how he is on the outside of this paradigm is a significant part of the narrator's progression and development. The struggle in being is a part of how the narrator understands his identity. The narrator sees struggle in recognition of how everyone around him, including his own brother, is different from him. For the narrator, identification of this aspect of difference in being is a significant part of his identity. It is an acknowledgement that displays his being alone in the universe. It is also a condition that recognizes the chasm between the individual and the public. In both of these realities, the presence of existentialism is a large one in Lu Xun's work.