四分律刪繁補闕行事鈔 2.439

2.439

所以爾者，昔解，若輕重相對，定有第三，故即成五。如淫戒，轉想及迷，並得夷蘭；若殺、妄、摩觸、二麁語等，轉想蘭吉，本迷亦吉。故須第三，以成五階。

The reason for this is: former interpretations held that if gravity and levity stand in opposition, there must definitely be the third, and therefore it forms five. For example, in the precept against sexual misconduct, whether one shifts ideation or is bewildered, both incur pārājika or thullaccaya; in killing, falsehood, physical contact, the two forms of coarse speech, etc., shifting ideation incurs thullaccaya or duṣkṛta, and original bewilderment also incurs duṣkṛta. Therefore, the third is required to complete the five grades.