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.
We have arranged for ourselves a world in which we can live - by positing bodies, lines, planes, causes and effects, motion and rest, form and content; without these articles of faith nobody could now endure life. But that does not prove them. Life is no argument. The conditions of life might include error.
Even great spirits have only their five-fingers' breadth of experience - just beyond it their thinking ceases and their endless empty space and stupidity begins.
It is not insult from another that causes you pain. It is the part of your mind that agrees with the insult. Agree only with the truth about you, and you are free.
If men would avoid that general language and general manner in which they strive to hide all that is peculiar, and would say only what was uppermost in their own minds, after their own individual manner, every man would be interesting.