The knowledge of sharia (sharia, the sacred law) is a single knowledge, encompassing within itself the two meanings known as transmission (riwāya) and understanding (dirāya). When these are combined, a knowledge of sharia emerges that calls to both outward (zāhir) and inward (bāṭin) acts. In this sense, it is not possible to consider zāhir and bāṭin as separate in knowledge, for knowledge resides in the qalb (the heart), and if it is not expressed by the tongue, it is considered ʿilm-i bāṭin (inward knowledge). When it is expressed by the tongue, it becomes zāhir.
For this reason, we do not say that knowledge is zāhir and bāṭin. In reality, knowledge is the knowledge of sharia that leads to both outward and inward acts, and necessitates both outward and inward acts. Outward acts pertain to the limbs and the body, such as purification, prayer, zakat, fasting, pilgrimage, as well as legal punishments, divorce, manumission of slaves, commercial transactions, inheritance law, retaliation, and similar rulings. All of these are outward actions performed with our bodies.
Inward acts, on the other hand, are acts of the qalb (the heart) and spiritual states and stations (ḥāl and maqām), such as affirmation, iman (faith), certainty (yaqīn), truthfulness (ṣidq), ikhlas (sincerity), maʿrifa (gnosis, experiential knowledge of God), tawakkul (reliance upon God), love (maḥabba), riḍā (contentment with God's decree), dhikr (remembrance of God), shukr (gratitude), ināba (turning to God), awe (khashya), taqwa (God-consciousness), muraqaba (spiritual vigilance), contemplation (tafakkur), reflection (iʿtibār), fear (khawf), hope (rajāʾ), sabr (patient endurance), contentment (qanāʿa), submission (taslīmiyya), consigning affairs to Allah (tafwīḍ al-umūr), nearness (qurb), longing (shawq), ecstasy (wajd), trembling (wajal), sorrow (ḥuzn), remorse (nadam), modesty (ḥayāʾ), shame (khajal), veneration (taʿẓīm), exaltation (ijlāl), and awe (hayba).
Each of the outward and inward acts has its own knowledge, comprehension, explanation, manner of understanding, ḥaqīqa (inner reality), and ecstasy (wajd). There are proofs for the soundness of every outward and inward act in the verses (āyāt) of the Qur'an and in the sunnah of the Prophet (peace and blessings be upon him). The one who knows these is a scholar; the one who does not is ignorant. (Lumaʿ, 43)
The inclination of the hearts of the people of the inward (bāṭin) is in proportion to the murīd's (aspirant, disciple) adherence to the noble sunnah and avoidance of innovations, and to the extent of his complete renunciation of his own existence. Indeed, among the leaders of this path, the common method is to negate existence, to exert much effort (gayra), to be content with what is at hand, to fulfill covenants, and, during the witnessing of the only true object of worship, the Mawlā (the Lord), not to recall anything other than Him. I recommend that you fulfill all of these as much as you are able, and that you persist in silent dhikr (remembrance of God) and in the awrād (regular litanies) we have summarized. For this is the most effective remedy for treating diseases of the qalb (the heart). The experts in unveiling (kashf) and the qalb also practice these. (Bughyat, 244)
Zāhir desires His rapid concealment; bāṭin desires His intense manifestation. (Nafahāt, 49)