首楞嚴義疏注經 4.319

4.319

初二句牒，本自下二句定。自，本也。然，是也。頭本如是，無有如是之頭不是於本，故云無然非自。何因下破，可知。

The first two lines reiterate; the two lines from "fundamentally of itself" downward establish. "Self" (zi) means fundamentally. "So" (ran) means being thus. The head is fundamentally like this; there is no such head that is not fundamentally so; hence it says, "there was nothing of its being so that was not of itself." From "due to what causes" downward refutes it, as can be understood.