Perhaps there is no other knowing than the mere competence of the act. If at the heart of one's being, there is no self to which one ought to be true, then sincerity is simply nerve; it lies in the unabashed vigor of the pretense. But pretense is only pretense when it is assumed that the act is not true to the agent. Find the agent.
...And these vicissitudes come best in youth;
For when they happen at a riper age,
People are apt to blame the Fates, forsooth,
And wonder Providence is not more sage.
Adversity is the first path to truth:
He who hath proved war, storm, or woman's rage,
Whether his winters be eighteen or eighty,
Has won experience which is deem'd so weighty.
...most men have bound their eyes with one or another handkerchief, and attached themselves to some one of these communities of opinion. This conformity makes them not false in a few particulars, authors of a few lies, but false in all particulars. Their every truth is not quite true. Their two is not the real two, their four not the real four; so that every word they say chagrins us, and we know not where to begin to set them right.