A Dirac fermion is equivalent to two Weyl fermions. The momentum of the fermions in the fermion gas nevertheless generates pressure, termed "degeneracy pressure". Each fermion has a corresponding antiparticle. Invariance under parity transformation by a Dirac fermion is called chiral symmetry. Invariance under parity by a Dirac fermion is called "chiral symmetry". It is not known whether the neutrino is a Dirac fermion or a Majorana fermion. These sequences terminate at the composite fermion Fermi sea. Several members of the group of heavy fermion materials become superconducting below a critical temperature. For example, the electron is an elementary spin-1/2 fermion. The theory of causal fermion systems is an approach to describe fundamental physics. The BCS theory is also applicable to other fermion systems, such as helium-3. This is the so-called Majorana fermion. Implementing this picture mathematically leads to the framework of causal fermion systems. In this section the mathematical framework of causal fermion systems is introduced. It also motivates the name causal fermion system.