 GST features a single primitive ontological notion, that of set, and a single ontological assumption, namely that all individuals in the universe of discourse (hence all mathematical objects) are sets.There is a single primitive binary relation, set membership; that set "a" is a member of set "b" is written "a"∈"b" (usually read ""a" is an element of "b"").The symbolic axioms below are from Boolos (1998