瑜伽論記 8.304

8.304

又解：性種姓體，總依菩薩內六處身，故言六處，雖爾，亦依於外六處身，然非有情，是非不定故，但言六內處。

Another explanation: The essence of the natural gotra generally relies upon the internal six-base body of the bodhisattva, hence it says 'six bases'. Although it also relies upon the external six bases, these are not sentient beings and their status is unfixed, hence it only mentions the internal six bases.