# XLIX explained — Philosophical Works > Chapter companion for Philosophical Works by René Descartes. ## What happens The chapter discusses eternal truths, such as 'ex nihilo nihil fit' and 'he who thinks must exist while he thinks,' which are not things or modes but common notions or axioms seated in the mind. It notes that while these truths are numerous and difficult to enumerate, such enumeration is unnecessary because they are self-evident when considered without prejudice. ## Themes in this chapter ### Reason and Method The chapter identifies eternal truths as axioms that are recognized by reason when free from prejudice, highlighting the method of clear and distinct perception. ### Skepticism and Certainty By asserting that eternal truths are self-evident and cannot be doubted when properly considered, the chapter establishes a foundation for certainty beyond skeptical doubt. ## Character check-ins ### Narrator The narrator presents the classification of eternal truths as common notions or axioms. ## Key lines > When we apprehend that it is impossible a thing can arise from nothing, this proposition, ex nihilo nihil fit , is not considered as somewhat existing, or as the mode of a thing, but as an eternal truth having its seat in our mind, and is called a common notion or axiom. The principle 'nothing comes from nothing' is not a physical object or property but an eternal truth that resides in the mind, known as a common notion or axiom. _This distinguishes eternal truths from substances and modes, placing them in the realm of innate ideas._ > he who thinks must exist while he thinks The act of thinking implies the existence of the thinker at the moment of thought. _This echoes the cogito and reinforces the certainty of self-existence during thinking._ ## Links - HTML: https://www.betterreads.online/discover/philosophical_works_se/chapters/xlix - Book: https://www.betterreads.online/discover/philosophical_works_se - All chapters: https://www.betterreads.online/discover/philosophical_works_se/chapters