Didn Descartes, with his cogito ergo sum, prove that at least some things are absolutely certain, e.g., that I exist?
A. Descartes thought he proved that it is absolutely certain that God exists and a whole lot of other things. See his Meditations on First Philosophy. Many of the claims that Descartes claimed were absolutely certain are referred to as analytic claims: their truth is a matter of convention and depends entirely on semantics or syntax, not empirical discovery. Other claims Descartes thought were absolutely certain, such as the claim that “God exists,” are neither absolutely certain nor analytic. “2+2=4″ is true for certain definitions of ‘2’, ‘+’ ‘=” and ‘4’ and false for other definitions, e.g., adding 2 raindrops to 2 raindrops does not give 4 raindrops, and mixing 2 liters of water with 2 liters of alcohol does not yield 4 liters of liquid. Words or signs rarely have meanings independent of other words and signs. Whether a statement is true or not depends on what it means, and what it means depends on its context—especially the context of its interconnectedness with other words or sig
