Existentialists have a tricky relationship with Descartes’ search for certainty. This search is a big part of his way of thinking, which is called rationalism. Descartes wanted to find undeniable truth through skepticism, but existentialists challenge this idea. They focus on personal experiences and the uncertainty of life.

In his famous work, "Meditations on First Philosophy," Descartes goes on a journey of doubt. He famously says, "Cogito, ergo sum," which means "I think, therefore I am." This phrase shows that he believes while we can doubt physical things, we cannot deny that we are thinking beings. Descartes breaks down belief systems that are uncertain. He aims to clearly understand reality using logic. His goal was to lay a strong foundation for science and philosophy.

On the other hand, existentialists like Jean-Paul Sartre and Simone de Beauvoir argue that Descartes’ focus on reason misses the messy reality of being human. They believe that being human isn’t just about rational thinking; it's about having freedom, making choices, and experiencing life personally. For existentialists, trying to find certainty is pointless because people create their own values and truths in a world that doesn’t provide them.

Key Points of Critique:

1. Subjectivity Over Objectivity:

   • Existentialists say that Descartes’ focus on objective thinking can't capture the richness of human life. Humans have feelings, values, and context, which are not just about rational thinking.
   • Sartre believes that focusing only on objective reasons ignores the personal freedom and responsibility that shape our lives.

2. The Role of the Absurd:

   • Existentialists highlight the absurdity of life, as explained by Albert Camus. Life doesn’t come with built-in meaning, which challenges the idea that certainty can ever be found.
   • While Descartes seeks certainty through logic, existentialists find freedom in accepting the absurdity of life. They believe that meaning must be created through personal choices.

3. Doubt as Existence:

   • For existentialists, doubt and uncertainty are not problems to fix; they are part of being human. The ongoing search for meaning reflects the complexity of life rather than a flaw in logical thinking.
   • This idea lines up with Kierkegaard’s belief that each person’s journey is subjective, and embracing existence often requires a leap of faith amid uncertainty.

4. Authenticity and Freedom:

   • Descartes’ approach can oversimplify human experience by putting rational thought above being true to oneself. Existentialists encourage authenticity, where people face their freedom and make choices that reflect who they really are, rather than just following societal norms.
   • Sartre warns against "bad faith." This happens when individuals ignore their freedom and responsibility, hiding behind fixed beliefs.

5. The Concrete Human Experience:

   • Existentialists want us to focus on real-life experiences, not just theories. They believe that things like suffering, joy, anxiety, and love are crucial to understanding what it means to be human. This contrasts with Descartes' abstract ideas focused on logic.
   • By looking at lived experiences, existentialists draw attention to human suffering and despair, which often get overlooked in Cartesian thinking.

In conclusion, existentialists challenge Descartes' quest for certainty, saying it ignores the complexities of life. They suggest we embrace uncertainty, personal experiences, and emotions to find meaning in life.

Where Descartes sought a solid base for knowledge from clear and distinct ideas, existentialists raise questions about that base. They highlight that being human is more about living and making choices than just knowing things. They believe that true existence involves navigating the uncertainties of life with bravery and authenticity instead of trying to remove doubt with logic.