Great article, why make a fundemental distinction between isa and association? any realtion can be modelled as isa. eg isa dog and isa agent who plays frisbee.

Great article, why make a fundemental distinction between isa and association? any realtion can be modelled as isa. eg isa dog and isa agent who plays frisbee. Any such relation can easily be modelled in a powerful relational database such as postgres, including recursive or ancestral, so why use this strange niche tool.