Thoreau isn't simply endorsing positive thinking—he's making a claim about *self-knowledge* as destiny. What matters isn't optimism but accuracy; a man who thinks himself capable of mediocrity will arrange his life accordingly, while one who honestly recognizes his own standards will find circumstances aligning with that recognition. The phrase "indicates, or rather indicates" reveals his shrewdness: he's suggesting that our self-conception doesn't magically create outcomes so much as it exposes what we were already capable of becoming. When someone abandons a difficult craft because they've decided they lack talent, they're not defying fate—they're simply making visible the limit they'd already set for themselves.