Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In mathematical set theory, a strongly compact cardinal is a certain kind of large cardinal number; their existence can neither be proven nor disproven from the standard axioms of set theory. A cardinal is strongly compact if and only if every -complete filter can be extended to a complete ultrafilter. Strongly compact cardinals were originally defined in terms of infinitary logic, where logical operators are allowed to take infinitely many operands. The logic on a regular cardinal is defined by requiring the number of operands for each operator to be less than ; then is strongly compact if its logic satisfies an analog of the compactness property of finitary logic. Specifically, a statement which follows from some other collection of statements should also follow from some subcollection having cardinality less than .