Agathist
An agathist believes that all things lead to good as opposed to optimist who believes that things are for the best, or 
pessimist who believes that things are for the worse.
Pessimism and optimism are judgements of a situation, which human beings find comforting because once we have decided, is a situation good or bad, we can 'place' ourselves in relation to the situation.
An agathist doesn't judge a situation good or bad, but simply accepts that it is.
An agathist only asks whether a situation serves him or not. If the situation does serve him he may choose to remain present. If it does not serve him he will choose to move towards a situation that better serves him or her.
In choosing to move away the agathist is learning