The philosophy of Søren Kierkegaard has been a major influence in the development of 20th-century philosophy, especially existentialism and postmodernism. Søren Kierkegaard was a 19th-century Danish philosopher who has been labeled by many as the "Father of Existentialism", although there are some in the field who express doubt in labeling him an existentialist to begin with. His philosophy also influenced the development of existential psychology.

Kierkegaard criticized aspects of the philosophical systems that were brought on by philosophers such as Georg Wilhelm Friedrich Hegel before him and the Danish Hegelians. He was also indirectly influenced by the philosophy of Immanuel Kant. He measured himself against the model of philosophy which he found in Socrates, which aims to draw one's attention not to explanatory systems, but rather to the issue of how one exists.

One of Kierkegaard's recurrent themes is the importance of subjectivity, which has to do with the way people relate themselves to (objective) truths. In Concluding Unscientific Postscript to Philosophical Fragments, he argues that "subjectivity is truth" and "truth is subjectivity." Kierkegaard conveys that most essentially, truth is not just a matter of discovering objective facts. While objective facts are important, there is a second and more crucial element of truth, which involves how one relates oneself to those matters of fact. Since how one acts is, from the ethical perspective, more important than any matter of fact, truth is to be found in subjectivity rather than objectivity.