I'm reading *[Mathematics: Its Content, Methods and Meaning](http://rads.stackoverflow.com/amzn/click/0486409163)*. The authors make what is to me a surprising claim:

> 
> To put it briefly, truth is concrete; and it is particularly
>  important to remember this fact with respect to mathematics, exactly
>  because of its abstractness.
> 
> 
> 

Is truth concrete? How is this claim justified?