首楞嚴義疏注經 1.366

1.366

破不遍也。若汝執心不遍身者，頭足之觸同時而下，一合有覺，一當不知。今汝咸知，孰成不遍？故云今汝不然。

This refutes 'non-pervasive.' If you hold that the mind does not pervade the body, then when the touches on head and foot land simultaneously, one should have awareness while the other should have no sensation. Now you feel both; how could it be non-pervasive? Hence he says, 'Now with you this is not so.'