In his history of philosophy, Frederick Copleston seeks to defend Descartes’s legacy against those who would argue that his methodical doubt is just an abstract attempt to arrive at certain knowledge. He writes:
The Cogito, ergo sum is therefore the indubitable truth on which Descartes proposes to found his philosophy… It is the first and most certain existential judgmenet. Descartes does not propose to build his philosophy on an abstract logical principle. In spite of anything which some critics may have said, his concern is not simply with essences or with possibilities: he is concerned with the existing reality, and his primary principle is an existential proposition.
–A History of Philosophy IV, 93
Copleston’s (and by extension, Descartes’s) problem, however, is twofold. First, Descartes objectified existence, so that even if he built his philosophy and his understanding of the nature of reality and God and knowledge on an “existential principle,” he abstracted himself away from the reality of that existential principle. This very fact, the foundation of his thought (methodical, relentless doubt) led him away from subjectivity, which, for Kierkegaard is truth. For Descartes, his own existential reality may have served as the foundation for the rest of his philosophical enterprise, but (like God himself) existence was no more than a pragmatic detail, an afterthought to objectified, rationalized knowledge. (In fact, Descartes himself meant to write a moral philosophy, but never felt he was able to do so. Not surprising, given his obsession with method and abstracted knowledge.)
Second, I find it unlikely that anyone can proceed upon Descartes’s project without feeling some level of despair about the amount of certainty that one can obtain about the nature of reality, knowledge, and how we ought to act. Further, embarking upon such a project necessarily forces humans (if they are honest about where the project has led them) to infinitely regress into skepticism and either hedonism or despair. Lack of certainty about anything but our very existence (which is the only “accomplishment” the modern epistemological project provides) is the only outcome. Thus, Kierkegaard says we are met with the paradox of God (the infinite) in time and faith, the vehicle of a good human existence.