Full Idea
The modern extensional notion of function is just an arbitrary correspondence between collections.
Gist of Idea
A function is just an arbitrary correspondence between collections
Source
Stewart Shapiro (Philosophy of Mathematics [1997], 1)
Book Reference
Shapiro,Stewart: 'Philosophy of Mathematics:structure and ontology' [OUP 1997], p.24
A Reaction
Shapiro links this with the idea that a set is just an arbitrary collection. These minimalist concepts seem like a reaction to a general failure to come up with a more useful and common sense definition.