Positive Space includes the images, text or other objects in you design. It is impossible to talk about, or take into consideration the positive space with out also including the negative space.
Negative space defines positive space, and visa versa.
Basically, one can not exhaust without the other. Spatial organization is very important in Design. Determining what will be your positive space verses your negative space is crucial to controlling your design. Not only do you have to take into consideration one page - but 2 or 3 at the same time. The image elements in this site are very nice.