Full Idea
When 2 + 2 =4 was first discovered, it was probably inferred from the case of sheep and other concrete cases.
Gist of Idea
Arithmetic was probably inferred from relationships between physical objects
Source
Bertrand Russell (Regressive Method for Premises in Mathematics [1907], p.272)
Book Reference
Russell,Bertrand: 'Essays in Analysis', ed/tr. Lackey,Douglas [George Braziller 1973], p.272