That (Self), when taught by an inferior man, is not easy to be known, even though often thought upon; unless it be taught by another, there is no way to it, for it is inconceivably smaller than what is small.

In plain words: If a poor teacher explains the Self, it is hard to grasp, even if you think about it a lot. You need a real teacher to show the way, because the Self is finer than the smallest thing.