Uncle Bob Does the set of all sets that doesn’t contain itself contain itself?
This statement is fale (logical loop)
Solution, restrict the types of a thing.
What is a type?
All that matters is the operations that can be performed on a type.
Bag of operations (outside in) < Same as class?
1988 - Barbara Liskov formal definition - subtypes can be used as their parent types
The representation rule - represetatives do not share the relationship of the things they represent.
E.g. Rect (height, width, area)
Square (height, width, area) are height and widths sides??
definition undefined behaviours = crash and burn