All 3 Uses of

subtle
in
Notes from the Underground

I saw it all, I divined it, but I did not dare to approach you first, because I had an influence over you and was afraid that you would force yourself, from gratitude, to respond to my love, would try to rouse in your heart a feeling which was perhaps absent, and I did not wish that ...because it would be tyranny ...it would be indelicate (in short, I launch off at that point into European, inexplicably lofty subtleties a la George Sand), but now, now you are mine, you are my creation, you are pure, you are good, you are my noble wife.†
Chpt 2.8 *

subtleties = things that are not obvious, but noticeable by someone with adequate sensitivity and relevant knowledge

It is so subtle, so difficult of analysis, that persons who are a little limited, or even simply persons of strong nerves, will not understand a single atom of it.†
Chpt 1.3

Have you noticed that it is the most civilised gentlemen who have been the subtlest slaughterers, to whom the Attilas and Stenka Razins could not hold a candle, and if they are not so conspicuous as the Attilas and Stenka Razins it is simply because they are so often met with, are so ordinary and have become so familiar to us.†
Chpt 1.7 *

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