 [Begr, Part III, §23:]  Through the present example, moreover, we see how pure thought, irrespective of any content given by the senses or even by an intuition a priori, can, solely from the content that results from its own constitution, bring forth judgements that at first sight appear to be possible only on the basis of some intuition. \(\ldots\) The propositions about sequences [\(R\)-series] in what follows far surpass in generality all those that can be derived from any intuition of sequences. [from the Bauer-Mengelberg translation in van Heijenoort 1967]