All 7 Uses of

subtle
in
The Scarlet Letter

...after living for three years within the subtle influence of an intellect like Emerson's;
p. 25.5 *

subtle = intelligent

Certainly there was some deep meaning in it most worthy of interpretation, and which, as it were, streamed forth from the mystic symbol, subtly communicating itself to my sensibilities, but evading the analysis of my mind.
p. 31.3

subtly = indirectly in a way that is hard to see

Dames of elevated rank, likewise, whose doors she entered in the way of her occupation, were accustomed to distil drops of bitterness into her heart; sometimes through that alchemy of quiet malice, by which women can concoct a subtle poison from ordinary trifles; and sometimes, also, by a coarser expression, that fell upon the sufferer's defenceless breast like a rough blow upon an ulcerated wound.
p. 79.3

subtle = working in an indirect or hidden way

In their researches into the human frame, it may be that the higher and more subtle faculties of such men were materialised, and that they lost the spiritual view of existence amid the intricacies of that wondrous mechanism, which seemed to involve art enough to comprise all of life within itself.
p. 110.4

subtle = requiring fine distinctions

All this was accomplished with a subtlety so perfect, that the minister, though he had constantly a dim perception of some evil influence watching over him, could never gain a knowledge of its actual nature.
p. 130.4 *

subtlety = working in an indirect or hidden way

The minister well knew—subtle, but remorseful hypocrite that he was!—the light in which his vague
confession would be viewed.
p. 133.9

We have had, and may still have, worse things to tell of him; but none, we apprehend, so pitiably weak; no evidence, at once so slight and irrefragable, of a subtle disease that had long since begun to eat into the real substance of his character.
p. 201.8

Definitions:

(1) (subtle as in: a subtle poison)
working in an indirect or hidden way
(2) (subtle as in: a subtle shade of blue)
understated so as not to draw excess attention
(3) (subtle as in: a subtle difference or thinker)
not obvious, but noticeable with enough insight and knowledge

or:

able to notice or understand things that require insight and sensitivity