The system of Commandments within the Torah is complete and absolute, independent of social consensus or shifting cultural trends [ביאור שטיינזלץ]. It functions much like a perfect living organism; any addition or subtraction creates a blemish that damages its divine perfection [מלבי״ם]. Consequently, individuals are forbidden from relying on their own intellect and logic to alter the details of the Commandments, even if their intention is to serve God more effectively [אבן עזרא, רבינו בחיי].
The primary approach among commentators is that the prohibition against adding refers to altering the defined structure of an existing Commandment. Classic examples include placing five scrolls in Tefillin instead of four, taking five species for the Lulav, or attaching five fringes to a garment [רש״י, רמב״ן, מזרחי]. This restriction also applies to a priest who decides to append his own personal blessing to the Priestly Blessing, despite his good intentions to increase the blessing upon the people [תורה תמימה]. Beyond structural changes, the prohibition encompasses inventing a new holiday or Commandment from one's own heart, as Jeroboam did [רמב״ן]. It also forbids performing a Commandment after its designated time has passed while explicitly intending to fulfill the obligation, such as sleeping in a Sukkah on the eighth day of the holiday [רמב״ן, גור אריה].
A deep conceptual link exists between adding and subtracting, revealing that every addition inevitably leads to a subtraction. In a practical sense, a person who attaches five fringes to a garment does not merely add an extra string; he invalidates the entire Commandment, thereby nullifying the original four [רא״ש, בכור שור, שפתי כהן, דעת זקנים]. On a psychological and spiritual level, the moment a person permits himself to add to the Torah, he essentially declares that the Divine standard is not absolute. Ultimately, this mindset will lead him to subtract from and completely abolish other Commandments [כלי יקר, תורה תמימה].
A major danger lies in the attempt to seek logical rationales for the Commandments and subsequently alter them based on human reasoning. King Solomon serves as a cautionary example of a leader who relied on his own wisdom; he assumed that since the reason for the prohibition against multiplying wives was to prevent a king's heart from going astray, he could take many wives without sinning. Yet, he ultimately failed [ספורנו, דברי דוד]. Commentators connect this peril to the historical sin of Baal Peor. Many Israelites participated in the worship of Peor not out of a desire to commit idolatry, but due to a severe intellectual error. They believed that defecating before the idol was a profound act of disrespect that would earn them merit. However, because this was the specific pagan method of worshipping that deity, they were punished severely. This tragedy demonstrates that God must not be served based purely on human intellect, but rather through exact obedience to His directives, without any addition or subtraction [כלי יקר, תולדות יצחק, צאינה וראינה, אלשיך].
In light of this strict prohibition, a question arises regarding how the Sages were permitted to enact new decrees, regulations, and holidays such as Hanukkah and Purim. Commentators explain that Rabbinic enactments are not considered prohibited additions because they serve as a protective fence around the original Commandments [רבינו בחיי, רלב״ג]. The crucial condition is that the Sages do not present these enactments as original Torah law, maintaining a clear and absolute distinction between a Torah prohibition and a Rabbinic decree [רמב״ן, גור אריה, רש״ר הירש]. In fact, the warning against adding is directed primarily at the High Court, cautioning them never to equate their own enactments with Torah laws regarding punishments and rulings [הכתב והקבלה]. The duty to safeguard the tradition entails a commitment to diligent study, which is the essential prerequisite for fulfilling the Commandments properly [מזרחי]. Ultimately, strict adherence to the prohibition against adding or subtracting preserves the original, pure character of the Commandments, ensuring they are observed out of absolute loyalty to God's will rather than an urge for human innovation [רש״ר הירש].