Q0055
“Once in a lifetime, everything must be doubted.” — René Descartes (Meditations on First Philosophy I)
Verified English quotation. Primary source: Meditations on First Philosophy I. Theme: knowledge.
English
Once in a lifetime, everything must be doubted.
Source and attribution
The archive attributes this English wording to Meditations on First Philosophy I and files it under Rationalism. That locator distinguishes the passage from loose internet attributions: readers can return to the named work or section to check translation, surrounding argument, and edition-specific wording.
About René Descartes
René Descartes (1596–1650) was a French mathematician and philosopher who helped launch early modern rationalism. Educated by Jesuits, he traveled as a soldier and settled for long periods in the Dutch Republic.
How to read this passage
Epistemology asks whether knowledge is justified true belief, reliable tracking of the world, or something else; it also asks how perception, testimony, memory, and reason contribute. This passage enters that discussion through knowledge, method, truth.
Concepts in this quote
- Knowledge
- Knowledge is the philosophical study of what we can justifiably claim to know—and where certainty ends.
- Method
- Method appears in this archive as a live philosophical concern—not a slogan. These verified English passages show how thinkers treat method as a problem of judgment, practice, or explanation.
- Truth
- Truth is among philosophy’s oldest problems: what it is for a claim to be true, and how truth relates to belief, language, and power.
How to cite this quotation
APA: René Descartes. (n.d.). Once in a lifetime, everything must be doubted. In Philosophy Blind Box. https://www.philosophydef.com/quotes/q0055
MLA: René Descartes. “Once in a lifetime, everything must be doubted..” Philosophy Blind Box, https://www.philosophydef.com/quotes/q0055.
Chicago: René Descartes. “Once in a lifetime, everything must be doubted..” Philosophy Blind Box. Accessed 2026. https://www.philosophydef.com/quotes/q0055.
Frequently asked questions
What is the source of the quotation “Once in a lifetime, everything must be doubted.”?
The passage is attributed to René Descartes, Meditations on First Philosophy I. The archive attributes this English wording to Meditations on First Philosophy I and files it under Rationalism. That locator distinguishes the passage from loose internet attributions: readers can return to the named work or section to check translation, surrounding argument, and edition-specific wording.
What does René Descartes mean by knowledge?
Epistemology asks whether knowledge is justified true belief, reliable tracking of the world, or something else; it also asks how perception, testimony, memory, and reason contribute. This passage enters that discussion through knowledge, method, truth.