Full Idea
The subject matter of mathematics is the concrete symbols themselves whose structure is immediately clear and recognisable.
Gist of Idea
The subject matter of mathematics is immediate and clear concrete symbols
Source
David Hilbert (On the Infinite [1925], p.192)
Book Reference
'Philosophy of Mathematics: readings (2nd)', ed/tr. Benacerraf/Putnam [CUP 1983], p.192
A Reaction
I don't think many people will agree with Hilbert here. Does he mean token-symbols or type-symbols? You can do maths in your head, or with different symbols. If type-symbols, you have to explain what a type is.