There is no way to obtain objective certainty with regard to existing realities other than myself. If we adopt the epistemic standards of modern classical foundationalism, we will become skeptics about the external world, and Kierkegaard thinks that Greek skepticism should have taught us this already. We arrive at the external world only through faith or belief. Nevertheless, we can arrive there. We can do so because we have a sense of what it means to exist in actuality, and we have such a sense because we know ourselves as actual agents.
Kierkegaard says that existence is not a concept, and hence it is incorrect to say that we learn the meaning of “existence” from our own case in the sense that we might learn the meaning of “white” by seeing white objects. Nevertheless, we do have a sense of what it means to exist, and we do make judgements about what things exist and what things do not, and the attitude Kierkegaard calls “belief” is an expression of this this distinction. One way of expressing this would be to say that one must analyze an individual’s belief about an independent reality as a linking of though-possibilities with that individual’s own existence. Though I have no concept of existence, I know what it means to exist by existing. Believing that my friend John exists amounts to linking John in some ways to that concrete actuality that is thought without becoming a mere possibility, namely, my own actuality. John is my friend, the one I went through high school and college with.
—C. Stephen Evans “Realism and antirealism in Kierkegaard’s Concluding Unscientific Postscript”