Full Idea
Just distinguishing things is not enough for counting (and hence arithmetic). We need the crucial extra notion of the successor in a series of some kind.
Gist of Idea
Counting needs to distinguish things, and also needs the concept of a successor in a series
Source
Michael Morris (Guidebook to Wittgenstein's Tractatus [2008], Intro.5)
Book Reference
Morris,Michael: 'Guidebook to Wittgenstein's Tractatus' [Routledge 2008], p.14
A Reaction
This is a step towards the Peano Axioms of arithmetic. The successors could be fingers and toes, taken in a conventional order, and matched one-to-one to the objects. 'My right big toe of cows' means 16 cows (but non-verbally).