成唯識論演祕 7.333

7.333

非四尋思與不定中尋相應故而得「尋」名。若尋相應方名尋思，尋思不應在第四禪，第四禪中無不定中尋法體故。

It is not because the four investigations are associated with vitarka among the indeterminate dharmas that they obtain the name 'investigation' (vitarka). If only that which is associated with vitarka were named investigation, investigation should not exist in the fourth dhyāna, because the real entity of vitarka among the indeterminate [dharmas] does not exist in the fourth dhyāna.