When telling the Pharisee that the second greatest commandment was to love one’s neighbor as oneself, Jesus was referring to a specific law given to Israel.
إِنَّ ٱلْأَسْفَارَ ٱلْمُقَدَّسَةَ تُسَاعِدُنَا عَلَى إِيجَادِ ٱلْجَوَابِ عَنْ هذَيْنِ ٱلسُّؤَالَيْنِ ٱلْمُهِمَّيْنِ .‏