Full Idea
Why are logical (or mathematical, or analytic...) truths necessary? The Humean's answer is that this is just how our concept of necessity works.
Gist of Idea
Humeans says mathematics and logic are necessary because that is how our concept of necessity works
Source
Theodore Sider (Writing the Book of the World [2011], 12.11)
Book Reference
Sider,Theodore: 'Writing the Book of the World' [OUP 2011], p.288
A Reaction
This is why I (unlike Sider) am not a Humean. If we agreed that 'necessary' meant 'whatever is decreed by the Pope', that would so obviously not be necessary that we would have to start searching nature for true necessities.