The spin symmetry in the Dirac sea has been investigated with relativistic Brueckner-Hartree-Fock theory using the bare nucleon-nucleon interaction. Taking the nucleus 16 O as an example and comparing the theoretical results with the data, the definition of the single-particle potential in the Dirac sea is studied in detail. It is found that if the single-particle states in the Dirac sea are treated as occupied states, the ground state properties are in better agreement with experimental data. Moreover, in this case, the spin symmetry in the Dirac sea is better conserved and it is more consistent with the findings using phenomenological relativistic density functionals.It is well known that in the nuclear system the spin symmetry is largely broken, that is, there exists a large spin-orbit (SO) splitting, which was introduced by Mayer [1] and Haxel et al. [2] in 1949. It formed the ground for the nuclear shell model. Twenty years later a new symmetry, the so-called pseudospin symmetry, was proposed to explain the near degeneracy between two single-particle (s.p.) states with the quantum numbers (n, l, j = l + 1/2) and (n − 1, l + 2, j = l + 3/2) [3,4]. The two states are regarded as the pseudospin doublets with the pseudospin quantum numbers (ñ = n − 1,l = l + 1, j =l ± 1/2).By starting from the Dirac equation, it was found that the angular momentum of the pseudospin doubletsl is nothing but the orbital angular momentum of the lower component of the Dirac spinor, and the pseudospin symmetry is exact when the sum of vector and scalar potential V + S vanishes [5]. The more general condition, d(V + S)/dr = 0, was proposed and can be approximately fulfilled in exotic nuclei [6,7]. The general condition for spin and pseudospin symmetry, namely that V + S is a constant for pseudospin symmetry is confirmed in Ref. [8] and its connection to spin symmetry was also suggested there. Since then, pseudospin symmetry has been realized as a relativistic symmetry and much work has been done to investigate its origin and its properties using phenomenological single-particle Hamiltonians, relativistic mean field theory, or relativistic Hartree-Fock (RHF) theory [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26].If one starts with a Dirac Hamiltonian, there exist single-particle states not only with positive energy but also with negative energy, states in the so-called Dirac sea. It was shown in Ref.[27] that the pseudospin symmetry in the positive spectrum has the same origin as the spin symmetry in the Dirac sea. In other words, the SO doublets in the Dirac sea has the quantum number (n,l, j =l ± 1/2), and the spin symmetry breaking term is proportional to d(V + S)/dr, similar to the pseudospin symmetry in the positive spectrum. The spin symmetry in Dirac sea has also been investigated intensively afterwards [28][29][30][31][32][33]. For comprehensive reviews on the study of pseudospin and spin symmetries, seeRefs. [34,35].Up until now, all the studies on the pseudospin symmetry in nuclei or the spin symmetry...