Sets initially seemed appealing because they are less abstract than types. However, the lack of abstraction turned out to be a fatal flaw in attempts to make

Sets initially seemed appealing because they are less abstract than types. However, the lack of abstraction turned out to be a fatal flaw in attempts to make sets the foundations of mathematics because: * sets cannot be rigorously defined using 1st-order ZFC * sets cannot be used to define higher-level abstractions of mathematics