I had been thinking about the plan of a Begriffsschrift for a long time before it assumed definite shape. The desire to exclude with certainty the assumptions tacitly introduced into the foundations of arithmetic led me to the Begriffsschrift of the year 1897. Occu?pation with the latter then led me to a more precise formulation of the basic concepts of arithmetic, although I can no longer give the precise details [of this]. The recognition that the bearer of a number is not a heap, an aggregate, a system of things but a concept was, I dare say, inevitably facilitated by the Begriffssch?rift. Instead of a concept we may also take its extension — or, as I also call it, the class that belongs to it. I believe I owe the distinc?tion between a heap (aggregate, system) and a class — which probably was not drawn sharply before me — to my Begriffsschrift, although perhaps you cannot detect any trace of this when reading my little book.