Kierkegaard has been called a philosopher, a theologian, the Father of Existentialism, both atheistic and theistic variations, a literary critic, a social theorist, a humorist, a psychologist, and a poet. Two of his influential ideas are "subjectivity", and the notion popularly referred to as "leap of faith". However, the Danish equivalent to the English phrase "leap of faith" does not appear in the original Danish nor is the English phrase found in current English translations of Kierkegaard's works. Kierkegaard does mention the concepts of "faith" and "leap" together many times in his works.
The leap of faith is his conception of how an individual would believe in God or how a person would act in love. Faith is not a decision based on evidence that, say, certain beliefs about God are true or a certain person is worthy of love. No such evidence could ever be enough to completely justify the kind of total commitment involved in true religious faith or romantic love. Faith involves making that commitment anyway. Kierkegaard thought that to have faith is at the same time to have doubt. So, for example, for one to truly have faith in God, one would also have to doubt one's beliefs about God; the doubt is the rational part of a person's thought involved in weighing evidence, without which the faith would have no real substance. Someone who does not realize that Christian doctrine is inherently doubtful and that there can be no objective certainty about its truth does not have faith but is merely credulous. For example, it takes no faith to believe that a pencil or a table exists, when one is looking at it and touching it. In the same way, to believe or have faith in God is to know that one has no perceptual or any other access to God, and yet still has faith in God. Kierkegaard writes, "doubt is conquered by faith, just as it is faith which has brought doubt into the world".
Kierkegaard also stresses the importance of the self, and the self's relation to the world, as being grounded in self-reflection and introspection. He argued in Concluding Unscientific Postscript to Philosophical Fragments that "subjectivity is truth" and "truth is subjectivity." This has to do with a distinction between what is objectively true and an individual's subjective relation (such as indifference or commitment) to that truth. People who in some sense believe the same things may relate to those beliefs quite differently. Two individuals may both believe that many of those around them are poor and deserve help, but this knowledge may lead only one of them to decide to actually help the poor. This is how Kierkegaard put it: "What a priceless invention statistics are, what a glorious fruit of culture, what a characteristic counterpart to the de te narratur fabula [the tale is told about you] of antiquity. Schleiermacher so enthusiastically declares that knowledge does not perturb religiousness, and that the religious person does not sit safeguarded by a lightning rod and scoff at God; yet with the help of statistical tables one laughs at all of life." In other words Kierkegaard says: "Who has the more difficult task: the teacher who lectures on earnest things a meteor's distance from everyday life -- or the learner who should put it to use?"
Kierkegaard primarily discusses subjectivity with regard to religious matters. As already noted, he argues that doubt is an element of faith and that it is impossible to gain any objective certainty about religious doctrines such as the existence of God or the life of Christ. The most one could hope for would be the conclusion that it is probable that the Christian doctrines are true, but if a person were to believe such doctrines only to the degree they seemed likely to be true, he or she would not be genuinely religious at all. Faith consists in a subjective relation of absolute commitment to these doctrines.
Kierkegaard's famous philosophical 20th century critics include Theodor Adorno and Emmanuel Levinas. Non-religious philosophers such as Jean-Paul Sartre and Martin Heidegger supported many aspects of Kierkegaard's philosophical views, but rejected some of his religious views. One critic wrote that Adorno's book Kierkegaard: Construction of the Aesthetic is "the most irresponsible book ever written on Kierkegaard" because Adorno takes Kierkegaard's pseudonyms literally, and constructs a philosophy which makes him seem incoherent and unintelligible. Another reviewer says that "Adorno is [far away] from the more credible translations and interpretations of the Collected Works of Kierkegaard we have today."
Levinas' main attack on Kierkegaard focused on his ethical and religious stages, especially in Fear and Trembling. Levinas criticises the leap of faith by saying this suspension of the ethical and leap into the religious is a type of violence (the "leap of faith" of course, is presented by a pseudonym, thus not representing Kierkegaard's own view, but intending to prompt the exact kind of discussion engaged in by his critics). He states: "Kierkegaardian violence begins when existence is forced to abandon the ethical stage in order to embark on the religious stage, the domain of belief. But belief no longer sought external justification. Even internally, it combined communication and isolation, and hence violence and passion. That is the origin of the relegation of ethical phenomena to secondary status and the contempt of the ethical foundation of being which has led, through Nietzsche, to the amoralism of recent philosophies."
Levinas pointed to the Judeo-Christian belief that it was God who first commanded Abraham to sacrifice Isaac and that an angel commanded Abraham to stop. If Abraham were truly in the religious realm, he would not have listened to the angel's command and should have continued to kill Isaac. To Levinas, "transcending ethics" seems like a loophole to excuse would-be murderers from their crime and thus is unacceptable. One interesting consequence of Levinas' critique is that it seemed to reveal that Levinas viewed God as a projection of inner ethical desire rather than an absolute moral agent. However, one of Kierkegaard's central points in Fear and Trembling was that the religious sphere entails the ethical sphere; Abraham had faith that God is always in one way or another ethically in the right, even when He commands someone to kill. Therefore, deep down, Abraham had faith that God, as an absolute moral authority, would never allow him in the end to do something as ethically heinous as murdering his own child, and so he passed the test of blind obedience versus moral choice.
Sartre objected to the existence of God: If existence precedes essence, it follows from the meaning of the term sentient that a sentient being cannot be complete or perfect. In Being and Nothingness, Sartre's phrasing is that God would be a pour-soi (a being-for-itself; a consciousness) who is also an en-soi (a being-in-itself; a thing) which is a contradiction in terms. Critics of Sartre rebutted this objection by stating that it rests on a false dichotomy and a misunderstanding of the traditional Christian view of God.
Sartre agreed with Kierkegaard's analysis of Abraham undergoing anxiety (Sartre calls it anguish), but claimed that God told Abraham to do it. In his lecture, Existentialism is a Humanism, Sartre wondered whether Abraham ought to have doubted whether God actually spoke to him. In Kierkegaard's view, Abraham's certainty had its origin in that 'inner voice' which cannot be demonstrated or shown to another ("The problem comes as soon as Abraham wants to be understood"). To Kierkegaard, every external "proof" or justification is merely on the outside and external to the subject. Kierkegaard's proof for the immortality of the soul, for example, is rooted in the extent to which one wishes to live forever.
Faith was something that Kierkegaard often wrestled with throughout his writing career; under both his real name and behind pseudonyms, he explored many different aspects of faith. These various aspects include faith as a spiritual goal, the historical orientation of faith (particularly toward Jesus Christ), faith being a gift from God, faith as dependency on a historical object, faith as a passion, and faith as a resolution to personal despair. Even so, it has been argued that Kierkegaard never offers a full, explicit and systematic account of what faith is. Either/Or was published 20 February 1843; it was mostly written during Kierkegaard's stay in Berlin, where he took notes on Schelling's Philosophy of Revelation. According to the Routledge Companion to Philosophy and Religion, Either/Or (vol. 1) consists of essays of literary and music criticism, a set of romantic-like-aphorisms, a whimsical essay on how to avoid boredom, a panegyric on the unhappiest possible human being, a diary recounting a supposed seduction, and (vol. II) two enormous didactic and hortatory ethical letters and a sermon. This opinion is a reminder of the type of controversy Kierkegaard tried to encourage in many of his writings both for readers in his own generation and for subsequent generations as well.
Kierkegaardian scholar Paul Holmer described Kierkegaard's wish in his introduction to the 1958 publication of Kierkegaard's Edifying Discourses where he wrote:
Kierkegaard’s constant and lifelong wish, to which his entire literature gives expression, was to create a new and rich subjectivity in himself and his readers. Unlike any authors who believe that all subjectivity is a hindrance, Kierkegaard contends that only some kinds of subjectivity are a hindrance. He sought at once to produce subjectivity if it were lacking, to correct it if it were there and needed correction, to amplify and strengthen it when it was weak and undeveloped, and, always, to bring subjectivity of every reader to the point of eligibility for Christian inwardness and concern. But the Edifying Discourses, though paralleling the pseudonymous works, spoke a little more directly, albeit without authority. They spoke the real author’s conviction and were the purpose of Kierkegaard’s lifework. Whereas all the rest of his writing was designed to get the readers out of their lassitude and mistaken conceptions, the discourses, early and late, were the goal of the literature. Edifying Discourses: A Selection 1958 Introduction by Paul Holmer p. xviii