  Recently we have reported that the lattice QCD can not study the physical
hadron formation from the unphysical quarks and gluons because it operates the
unphysical QCD Hamiltonian of all the partons inside the hadron on the physical
energy eigenstate of the hadron to obtain the physical energy eigenvalue of the
hadron. However, since the parton distribution function (PDF) inside the hadron
is unphysical (although it is well defined in QCD), we find in this paper that
the unphysical energy of all the partons inside the hadron (instead of the
physical energy of the hadron) can be used to study the PDF using the lattice
QCD. Hence we find that the lattice QCD can study the parton distribution
function inside the hadron from the first principle.
