Summary: We are currently discussing the trait of Tahara (purity), which the Ramchal has defined as the rectification of the mind, as opposed to Prishus (abstinence) which is the rectification of actions. It is not enough to be refined in our behaviors; the intentions behind our actions need to be pure.
The Ramchal continues: "He who is truly a servant of Hashem will not be satisfied with serving Him minimally. He will not be content to take silver mixed with dross and lead, that is, Divine service mixed with impure motivations, but rather only with the clean and pure as is befitting. Then he will be called ‘one who does a Mitzvah as it is stated’, of which Chazal said (Shabbos 63a): ‘whoever does a Mitzvah as it is stated, will receive no evil tidings’ Likewise they said: ‘do [good] deeds for the sake of their Maker, and speak of them for their own sake’ (Nedarim 62a).” (Mesilas Yesharim, Ch.16)
"Dross" refers to impurities found in precious metals. Left alone, these compromise the strength and beauty of the material. It is interesting to point out that dross is removed by heating (whatever does not burn up separates from the melted silver and can be scooped away).
Similarly, there must be an intense process of burning our selfish motivations and ego until we get to the real purpose of our Mitzvah performance – for the sake of Hashem. This is often a multi-step process that takes a very long time, but as long as one is trending in the right direction, that is all Hashem wants.
