RAMBAM’S Sixth Principle: A mitzvah’s positive and negative commands are counted separately

Just as coins have two sides, some things are both a positive mitzvah if one does A and a negative mitzvah if one does B. There are numerous examples: Refraining from labor on Shabbos is a positive mitzvah; performing labor violates a negative mitzvah. Letting the ground rest during Shemittah (the Sabbatical year) is a positive mitzvah; working the land violates a negative mitzvah. Fasting on Yom Kippur is a positive mitzvah; eating on that day violates a negative mitzvah. This is not self-evident: failing to give charity or to put on tefillin does not violate a negative mitzvah, it merely fails to fulfill a positive mitzvah. Similarly, one does not fulfill a positive mitzvah every time he refrains from committing murder (which, hopefully, is always), he merely violates a negative mitzvah for its transgression.