“Dialectic” or “dialectical thinking” is an important topic that permeates Kierkegaard's philosophical anthropology as a whole. In this paper, I would like to clarify which way of thinking the dialectic that Kierkegaard says in a positive sense refers to, and how the movement of dialectic thinking is implemented in his anthropological observation and analysis. This paper attempts a detailed interpretation of Chapter 1, Section 5 of his work The Concept of Anxiety (1844). Kierkegaard's spirit is not a certain mental ability, but a “self-related (referential) movement” in which he produces his own other and tries to establish a relationship with this other in order to confirm his actual reality. This “synthesis of the spirit” is an act of combining two conflicting terms (soul/body, infinite/finite, possibility/necessity, time/eternality) by spontaneously establishing a relationship with each other. At this time, the nature and meaning of the two terms are materialized and determined only by the free action of the synthesis. Kierkegaard clearly argues that the self-related movement of the spirit and the object of anxiety undergo a temporal (historical) and qualitative change. In a word, the change of the spirit proceeds by fully embodying a form of dialectical thinking called “defined negation.” In addition, the spirit is aware that the basis for its existence and the conditions of the possibility of freedom are beyond its own power. The spirit cannot define and control itself completely transparently. However, nevertheless, the spirit must live its own life while enduring the difficult task of dialectical synthesis, which is becoming “one itself as well as a member of the historical community.”