In A_Treatise_of_Human_Nature, of David Hume, it is written:
For if truth be at all within the reach of human capacity, it is certain it must lie very deep and abstruse: and to hope we shall arrive at it without pains, while the greatest geniuses have failed with the utmost pains, must certainly be esteemed sufficiently vain and presumptuous.
I gathered that the usage seems to be to lie deep instead of to lie deeply. Why does an adjective follow the verb lie instead of an adverb? Is lie here a linking verb? Would the phrase It is certain it must lie very deeply and abstrusely. be incorrect?
