大般涅槃經義記 1.142

1.142

「何以」下釋，初先徵問。問有兩意：一問行因，自能得果，我以何故云言施汝？二問五果，是有為法，我以何故言施汝常？下別釋之，先解施常。釋意如何？由我常故，能施汝常。然今文中但解佛常，佛常既成，彼常自顯，不勞更辯。

From 'why' onward is the explanation; first it inquires. The inquiry has two meanings: first, asking that if cultivating causes naturally enables one to attain the fruit, for what reason do I say that I bestow it upon you? Second, asking that if the five fruits are conditioned dharmas, for what reason do I say that I bestow upon you what is eternal? Below it explains them separately, first explaining the bestowal of the eternal. What is the intention of the explanation? Because I am eternal, I can bestow the eternal upon you. However, the present text only explains the Buddha's eternity; once the Buddha's eternity is established, his eternity becomes evident on its own, and there is no need to deliberate further.