法苑珠林 4.65

4.65

依起世經云。佛告諸比丘。或有外道來問汝云。何因緣故虛空中忽生電光。汝應答云。有二因緣雲中出電。何等為二。一東方有電名曰無厚。南方有電名順流。西方有電名墮光明。北方有電。名曰百生樹。二者或有一時東方所出無厚大電與彼西方墮光明電相觸相對相磨相打。以如是故從彼虛空雲聚之中。出生大明。名曰電光。或復南方順流大電與彼北方百生樹大電相觸相對相磨相打。以如是故出生電光。譬如兩木風吹相著。忽然火出還歸本處（依經或先有雷無電。或先有電後雷。相擊火出霹靂人物也）。

According to the Lokotpatti-sūtra: The Buddha told the monks, 'If non-Buddhists come and ask you, "By what cause and condition does lightning suddenly flash in the sky?", you should reply, "By two causes and conditions lightning issues from the clouds. What are the two? First, in the east there is lightning called Thickness-less; in the south there is lightning called Along-the-Current; in the west there is lightning called Falling Light; in the north there is lightning called Hundred-Born Tree. Second, at times the great Thickness-less lightning issuing from the east and that Falling Light lightning of the west collide, face each other, rub against each other, and strike each other; for this reason, from within the cloud-masses in the sky there arises a great illumination, which is called lightning. Or again, the great Along-the-Current lightning of the south and that great Hundred-Born Tree lightning of the north collide, face each other, rub against each other, and strike each other; for this reason lightning arises. It is just as when two pieces of wood blown by the wind strike together, fire suddenly emerges and returns to its source."' (According to the sūtras, sometimes there is first thunder without lightning, or first lightning and then thunder; striking each other, fire issues forth and strikes down people and things.)