A set is hereditarily countable when its transitive closure is countable. These sets form the transitive set and all have rank below .
The transitive set of hereditarily countable sets satisfies every ZFC axiom except the Axiom of power set. For internal definable functions on a countable domain, ambient Axiom schema of replacement gives a countable range of hereditarily countable sets, which belongs to the structure by countable-subset closure. The same closure proves axiom schema of collection and retains countable choice-function graphs. All subsets of are elements, but their uncountable full power set is not an element, so Power Set fails.
A countable well-founded relation that is an extensional relation codes a transitive set through the Mostowski collapse theorem. Adding a distinguished point selects one decoded set. Every hereditarily countable set has such a code by enumerating its transitive membership ancestry. There are only continuum many codes. In ZFC, selecting least codes in a well-order gives an injection of the hereditarily countable sets into a set of size .
A set has countable-subset closure when every countable subset of it is one of its elements, including the empty subset. In ZFC, the hereditarily countable sets form the least such set. First realize them as the separated subset of with countable transitive closure. Countable unions show closure; well-founded induction proves leastness because each of these sets is a countable collection of earlier members.
A set is reasonable when every member of is countable. In ZFC this is equivalent to being hereditarily countable.
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A set is called **hereditarily countable** if it is countable, and all of its elements (and their elements, recursively) are also countable. In more formal terms, a set \( A \) is hereditarily countable if: 1. \( A \) is countable. 2. Every element of \( A \) is countable. 3. Every element of every element of \( A \) is countable, and so on.