Why use a primary text in philosophy? A primary text in philosophy is one that was written by the philosopher or person who thought of and wrote down an idea. Therefore, if you are studying Immanuel Kant, you will probably want to read texts written by Kant himself, in addition to reading articles and books that were written about Kant's philosophy.
While many of these primary texts are available in books, we also have large collections of primary texts in databases online. These are easy to find and some are even translated in English or French (keep in mind that many primary texts are written in Latin or German, or any number of other languages).