The elephant in the room in Descartes’ meditations is whether or not we can claim anything to be true.
For codicalists (and other pragmatists), what is (computes to be) ‘true’ (or ‘false’, etc.) is an expression of a particular (coding) language (with its own rules of judgment) that one is using for some purpose, not some entity that is independently, eternally true. That includes ‘2+2=4’ and ‘It is raining outside’.
In the codicalist — the intercourse of linguistic pragmatism and substrative transcendentalism — view, what’s beyond the limits of our babel of languages is there, but we will have continuing doubts of what that there is.
Note. Codicalism thus could be compared with the OFS of Markus Gabriel: “The ontology of fields of sense (OFS) is committed to a combination of ontological pluralism, ontological realism and metametaphysical nihilism. It is a view of reality according to which all sorts of things are real (in their respective fields of sense) without there being a single reality to which all real things belong.” Whether the substrative there of codicalism, in contrast, would be a singular or pluralistic reality is an issue, but the codicalistic there is independent of human beings.
At this point I think that Markus Gabriel’s “fields of sense” (cf. Fields of Sense: A New Realist Ontology by Markus Gabriel, Reviewed by Tom Sparrow) is, from a codicalistic view, another confusing of (our babel of) languages with substrate: sense fields is just another language (instrument) invented to try to make sense of the extralinguistic (extracodical, noumenic) substrate (reality) of nature.