Muhammad Taqi Al-Majlisi writes:

وَالظَّاهِرُ أَنَّ الْمُهَاجَرَةَ فِي الدِّينِ عِبْارَةٌ عَنْ تَغَرُّبِهِ لِطَلَبِ الْعُلُومِ الدِّينِيَّةِ أَوْ لِلْعِبَادَةِ أَوْ لِلْمَذْهَبِ وَالْمُرَادُ بِالْفِقْهِ الْعِلْمُ مُطْلَقًا وَالْمَعْنَى الْمُصْطَلَحُ لَمْ يَكُنْ فِي زَمَنِ الْأَئِمَّةِ صَلَوَاتُ اللهِ عَلَيْهِمْ عَلَى الظَّاهِرِ فَإِنَّهُمْ عَلَيْهِمُ السَّلَامُ كَانُوا يَنْفُونَ الْاِجْتِهَادَ وَالتَّقْلِيدَ كَمَا هُوَ ظَاهِرٌ لِلْمُتَتَبِّعِ وَالْمُرَادُ بِالْعَقْلِ آثَارُهُ مِنَ التَّدَيُّنِ بِدِينِ الْحَقِّ أَوِ الْعَمَلُ الصَّالِحُ كَمَا نُقِلَ عَنْهُمْ عَلَيْهِمُ السَّلَامُ إِنَّ الْعَقْلَ مَا عُبِدَ بِهِ الرَّحْمَنُ وَاُكْتُسِبَ بِهِ اَلْجِنَانُ وَكَثِيرًا مَا يُطْلَقُ عَلَى الْمَعْنَى الْأَوَّلِ كَمَا هُوَ الظَّاهِرُ لِلْمُتَدَبِّرِ فِي الْأَخْبَارِ

“Emigration (Muhajarah) for the sake of religion is obviously a way of expressing his going abroad to seek religious knowledge or to turn to worship or denominational aspects, and understanding (Fiqh) refers entirely to knowledge and the terminological meaning (jurisprudence) obviously did not exist in the time of the Imams (a.), for they used to reject effort (ijtihad) and imitation (taqlid) as it is clear for the one who investigates the facts, and by reason (aql) is meant his impression of the practice of the religion of truth or good conduct, as reported by the Imams (a.): “Reason (aql) is that by which one worships the Most Gracious and that by which one acquires the Gardens of Paradise.” And many of that which is used in the first meaning, as it is clear to the one who is thoughtful of the traditions.” [Rawdat-ul-Muttaqin, Volume 3 Pages 93 – 94]