So, basically, collision detection? Or did you mean something else? I think each entity should provide information about itself (like its geometry, perhaps via vertices) to some collision detection subsystem which will work out what collides via some algorithm. There are many available, for instance there is an algorithm to detect if two polygons collide in two dimensions (google it), and for other more analytical shapes like circles there should be formulae available too, though you can polygonize them if all else fails.

So, basically, collision detection? Or did you mean something else? I think each entity should provide information about itself (like it's geometry, perhaps via vertices) to some collision detection subsystem which will work out what collides via some algorithm. There are many available, for instance there is an algorithm to detect if two polygons collide in two dimensions (google it), and for other more analytical shapes like circles there should be formulae available too, though you can polygonize them if all else fails.