What is the significance of Descartes’s statement, cogito ergo sum, and how does his conclusions regarding the human soul overturn centuries of philosophical thought?

Book assigned for the course: “Meditations on First Philosophy”, Descartes (Hackett)

To achieve clarity in writing, you must ask yourself three questions: (a) Are all of the terms (used by the author or myself) clear in their meaning? (b) Are all of the propositions (used by the author or myself) necessarily true or perhaps dubitable in some way? (c) Are all of the arguments (given by the author or myself) both valid and sound?3.  To achieve writing that is concise, you must constantly ask yourself: (a) Are some of my sentences repetitive? (b) Am I relying on quotes more than I am explaining, when possible, in my own words? (c) Does each sentence build upon those before it, and does my writing attempt to move efficiently towards a conclusion?a.  On citations: quote if necessary, but try to formulate ideas in your own words. If you are discussing an authors ideas, you should always cite the source of these ideas by giving page numbers or Bekker numbers (the citation format for Aristotles texts). The aim of these papers is to practice your writing and thinking, rather than your ability to quote and cite material correctly.b.  For citations: your preferred style (e.g., MLA or Chicago) is fine. The important point here is simply to be consistent and complete.