The legal causes and reasons are divided into the following categories:1- Subject (mawḍūʿ) of legal rulings 2- Things which replace the cause (ʿilla) and the wisdom (ḥikma) As for the first type, the ruling (ḥukm) ceases to exist when the subject matter is nonexistent. When it comes to the second type, it is a matter of debate and contention, and there are three types of it:1- Anything that replaces the cause and the wisdom 2- The cause itself from the previous type, if the ruling ceases to exist with the nonexistence of the cause 3- The cause from the previous type if the genera of the ruling cease to exist with the nonexistence of the cause.And the subject of research is the last two types, i.e. the second and the third type, and each of them has more than fourteen types and rulings, except after distinguishing the causes from the wisdom in practice. Al-Muḥaqqiq al-Nāʾīnī (may God have mercy on him) offers a different definition which is not to be underestimated, and the investigations of this treatise revolve around it.