My dissertation has two main parts. In the first half, I draw out an underlying presupposition of Descartes' philosophy: what I term "atomism of thought." Descartes employs a radical procedure of doubt in order to show that the first principle of his philosophy, the cogito, is an unshakeable foundation of knowledge. In the dialogue that follows his dissemination of the Meditations, Descartes reveals that a whole set of concepts and rational principles innate in our minds are never doubted. These fundamental units of thought are indivisible, distinct, and isolated, and enable the possibility of any rational demonstration. Atoms of thought are perfectly individuated because God has created them as such. Likewise, our minds have been fashioned such that we necessarily have a clear and distinct perception every time we alight upon these simple notions. In the second part of the dissertation, I take up critiques of Descartes' view given by Nietzsche and Heidegger. In the chapter on Nietzsche, I attempt to fill a lacuna in scholarship about Nietzsche's commentary on Descartes. More specifically, I argue that once the foundation of God is displaced, the basis for accepting atomism of thought dissolves. In the final chapter, I analyze Heidegger's critique of Cartesian atomism. I first look at Heidegger's critique of classical truth as correspondence from Being and Time, and show how it is relevant to a critique of atomism. Then I show how the early Heidegger's holistic philosophical framework can provide an alternative that avoids the pitfalls of atomism. While I limit the scope of my analysis to Descartes' particular formulation, atomism of thought was an influential doctrine throughout modern philosophy. This aspect of Cartesianism has persisted and continues to be a significant theoretical underpinning of many contemporary views. It is my contention that Nietzsche and Heidegger have important contributions to make to this area of thought, and the relative neglect of their work in recent scholarship is a detrimental oversight.