Everything in this world is a product of cause and effect, R Salanter points out in this letter (all the way back of course to the original Cause, G-d Almighty, he adds). We all know how true that is when it comes to the natural world, but wed more likely succeed if wed remember it when it comes to our spiritual growth, too, as well see.
We know, for example, that were often personally motivated (i.e., caused) to succeed materially by some external prods, and that the more momentous the motivation, the greater the effect. So, lets see how to take advantage of that spiritually.
R Salanter then returns here to the themes of Rosh Hashanah and Yom Kippur that hed addressed in his previous letter (since this one was also written at that time of the year). He says that the classical impetus and cause of change in that season has been the blowing of the Shofar. But since our hearts are (so) unfeeling and as hard as stone, in our day, thats hardly enough to move us to Teshuvah. So, wed do well to also make regular trips to a Mussar House and to study there assiduously in order to be prompted to repent. Doing that, will yield significant and vital results, as it would more likely touch the core and act as a strong motivation.
And he encourages us to especially study Mussar before Rosh Hashanah arrives and when were most especially advised to humble ourselves. His point is that its easy enough to become humble (as well as motivated to change) at that time when we consider that many people are apparently sentenced to die (G-d forbid) in the year ahead on Rosh Hashanah itself, since they die in the course of that year.
So, we should at least resolve to improve ourselves on some minimal level before approaching Yom Kippur by determining to study Mussar regularly and reflecting on where we need to improve ourselves at the very least.
Now, all of thats true, but it might seem off-putting, the truth be known, and discouraging. Understanding that, R Salanter encourages us to never give up hope as theres no affliction as serious as despair he underscores. So, in order to salvage our pride and sense of optimism about our spiritual stature lets try to to (at least) fulfill the mitzvos that are easy to, and avoid the sins that are easy to avoid. And we will have succeeded on many levels that way.