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Achilles / turtle / Carroll / Nozick: logical form: (1) If p then q - (2) p.
Problem: q is not yet accepted, but it is required that the following will also be accepted explicitly:
(3) If (if p then q) and p, then q - regress: the additional assumption (3) (s) in addition to the modus ponens) therefore is: if (1) and (2), q.
This then in turn needs an additional premise.
Solution / tradition: Problem: confusion of premises with inference principles. Then the regress not even begins. - Solution / Nozick: we need to introduce a premise that has the same shape and all inferences supplies as the other assumptions that are apparently still needed - WittgensteinVs: problems of rule-following, etc.
Philosophical Explanations Oxford 1981
The Nature of Rationality 1994