Why largely?

In an investigation when new clues appear, we know we are moving in the right direction; that is, that what we have done so far in the investigation is largely correct. This is what the Right Direction Catechism tells us.

Some will ask, why the 'largely'? Are we hinting there could be mistakes in some of the early steps even though new clues appear?

This is indeed the case. Even though new clues appear, our interpretation of some of the old clues could be wrong. The point to notice here is, the formation of new clues does not always depend on all the old clues. If enough of the old clues have been correctly interpreted new clues could appear; it does not have to be the case that all the old clues have to be correctly interpreted. In the English language the letter X is not used all that often. Suppose in cryptanalyzing a message we make a mistake in deciphering X. Suppose X occurs only once in this message. Now when this is the case, the mistake could easily pass unnoticed. The one time that X occurs we make a mistake in interpreting it. But X does not occur again; it therefore cannot contribute to the formation of new clues. If the investigation nevertheless advances, it will be due to other clues. Even if X had been correctly interpreted, it still would not contribute to the formation of new clues since it occurs only once and does not occur a second time.