In Discourse on Method (DOM), Descartes wrote a history of his own philosophical pilgrimage and a program for his philosophical and scientific system. The DOM serves as a programmatic summary of what Descartes considered a revolutionary new method of knowledge; it is a kind of philosophical confession of faith which serves as an apology.
It is important to remember that DOM was the preface to three works of science--the Geometry, the Dioptrics, and the Meteorics--which followed it in the original text published in 1637. These served as proof of the success of the method.
Descartes presents himself as an earnest and humble seeker of truth. He delights in all manner of learning, but is unsatisfied with its uncertain status. Though well-schooled and well-traveled as a young man, his heart finds no rest in authority and tradition. He finds clarity and certainty in mathematics but sees little of philosophical value built upon its foundations. So he is discontent.
It is important for Descartes to convince the reader that he failed to find certainty in the conventional places. This helps to convince the reader that a new and revolution method is needed. Descartes now resolves to make himself alone the object of study.
In Germany, Descartes repairs to a secluded room heated by a stove to occupy his attention with his own thoughts. Here he begins to discover his method.
When Descartes begins to philosophize he discovers four "precepts of logic" which he resolves never to violate: (1) to believe nothing except what is clear and distinct, (2) to divide up problems into appropriate parts, (3) to proceed from the simple to the complex, and (4) to make sure nothing is left out. This method can, Descartes thinks, deliver a kind of mathematical certitude. Yet he deems himself too young and inexperienced to commence the project. He waits until he is more mature.
Descartes then sets out to doubt all that can be doubted (while keeping religion provisionally intact), not in order to be a skeptic but to find indubitable certainty. He then gives a brief treatment of issues more thoroughly addressed in (also autobiographical) The Meditations (published later). Everything can be doubted except himself as a doubter: "I think, therefore I am" becomes the indubitable and foundational principle of certitude upon which the structure of knowledge and science can be safely built.
He goes on to report his conclusions concerning the existence of God, the distinction of mind and body and the immortality of the soul. These conclusions are reached by rebuilding philosophy from himself as an autonomous knower. Thus, according to Hirom Caton, is "the origin of [modern] subjectivity" and the animus of his autobiographical stance. (I owe several of the following observations to Caton's book, The Origins of Subjectivity.)
To this point, Descartes uses philosophical autobiography quite skillfully. The telling of a story draws in readers who might not be normally attracted to philosophy. He conveys a sense of intellectual adventure and he personalizes the positions as his positions, grounded on his perspective, not merely as abstract speculations of no one in particular.
The autobiographical form is not incidental to the philosophy proposed. Descartes' autobiographical form is intrinsically connected to his philosophical program. Because of his distrust of tradition, he must, as it were, begin philosophy all over again from himself as an autonomous being. The starting point for Descartes' positive philosophy is not an abstract proposition but an existential statement, "I think, therefore, I am." By making himself the central object of study, he demonstrates that philosophy and autobiography are intimately and necessarily related. A statement of his philosophy requires a history of the study of himself.
Descartes used autobiography to avoid an overtly didactic or dogmatic manner by saying that the book is merely a record of how he himself conducts his reason. Using the autobiographic form, Descartes can avoid the authoritarianism of the schools while still making broad sweeping claims as to the veracity and utility of his method. He believes his method is sound, but he asks his readers to test what he says against their own reason. He thus decided to write the work in the "vulgar" French instead of the traditional--and scholastic--Latin.
The rest of DOM consists of portions of an unpublished work on physical science and Descartes' comments on why he has not published it. In these later sections Descartes launches into a kind of campaign speech for his own greatness and the indispensability of his scientific method.
He laments that he could not publish his work--The World--because of the prevailing authority's disagreements with his conclusions. Descartes fears that publication would threaten the equanimity he needs to be a successful seeker of truth; but his dedication to human betterment demands that he arrange for its posthumous publication.
Descartes is particularly shrewd at this point. He seems to be enlisting public support for the publication of a work he is afraid to publish. He tantalizes the reader by saying that his findings have tremendous practical value for the alleviation of human suffering and betterment of human health. Descartes ends the DOM with a restatement of his mission as a seeker of truth and benefactor of humanity.
All in all, Descartes uses philosophical autobiography quite successfully. The form fits the philosophy throughout most of the DOM in that Descartes is challenging the inherited wisdom of the schools by urging a personal discovery of truth through a radical method of rethinking philosophy with self-awareness at the center. His self-awareness is the foundation of his method.
Descartes presents himself as a kind of philosophical hero of almost mythical dimensions who is on a quest for the holy Grail of certainty. He claims to have captured the Grail which blesses him with the ability to envision an entirely new philosophy and science of nature. He gives the Geometry, the Dioptrics, and the Meteorics (which follow the DOM) as positive evidence of the fruitfulness of his method. They are, he claims, harbingers of even greater discoveries.
Nevertheless, Descartes sustained self-promotion at the end of the work seems intrusive at times and does little actual philosophical work. He is fighting for the chance to be heard by showing us his good intentions and intellectual genius. But the philosophical hero succumbs to hubris and the semblance of humility is betrayed by a kind of egotism which spills over the bounds of propriety and actually gets in the way of concrete philosophizing.