四分律行事鈔資持記 3.560

3.560

三中。防是能觀，心即所觀。此為觀法之本，前後四觀，止為防心，故處乎中以統前後。

In the third: "Guarding against" is the contemplating subject, and "mind" is the contemplated object. This is the root of the method of contemplation. The four contemplations before and after exist solely to guard against the mind; therefore, it is placed in the center to govern what precedes and follows.