elusive, subtle -- (difficult to detect or grasp by the mind or analyze; "his whole attitude had undergone a subtle change"; "a subtle difference"; "that elusive thing the soul")
subtle -- (able to make fine distinctions; "a subtle mind")
insidious, pernicious, subtle -- (working or spreading in a hidden and usually injurious way; "glaucoma is an insidious disease"; "a subtle poison")
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