subtle

adjectives

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"
able to make fine distinctions
"a subtle mind"
is similar to
having the ability to perceive or understand; keen in discernment
"a perceptive eye"; "a perceptive observation"
working or spreading in a hidden and usually injurious way
"glaucoma is an insidious disease"; "a subtle poison"