It is not the dead person who makes impure, nor is it the water that purifies, but the Holy One, blessed be He, said, "I have established a rule, I have decreed a decree, and you have no permission to transgress it, for it is written, "This is the rule of the Torah"” (Numbers 19:1). “We see that, although Parashat Chukat specifically refers to the laws of the impurity of a dead person, nevertheless our sages learn from it the general concept of purity and impurity, as Maimonides says, ‘It is clear that purities and impurities are decrees of the Torah, and are not among those matters that the intellect of man comprehends’” (end of Laws of Mikveh).
The Torah is eternal, therefore it has a teaching for us in all times and in all places. Even those precepts that only apply during the time when the Great Temple of Jerusalem was built, their physical application is limited by time, but their spiritual meaning is eternal and forever relevant.
Parashat Chukat begins with the laws of the Red Heifer, which were used in the time of the Holy Temple to purify a person who had become ritually impure through contact with a dead body. When explaining these laws, the Torah begins with "This is the rule of the Torah," rather than saying "This is the rule of the heifer," or "This is the rule of purity," etc. This signifies that the rule of the Red Heifer contains the general rule of the entire Torah; hence the phrase "This is the rule of the Torah.".
The essence of the Divine commandments (mitzvot) is that they are the Divine Will, superior to wisdom and reason. Even those commandments that have a rational explanation, the essence of the commandment is not the rational explanation itself, but rather the Divine Will that is clothed in a logical explanation understandable to us; that is its essence.
Therefore, the fulfillment of the precepts, even those that have a logical explanation, should not be done because the intellect dictates it, but because it is the Divine Will.
That is why it is written "This is the rule of the Torah," because all the precepts of the Torah are truly rules, decrees of the Divine Will.
The will is indivisible. Whether it's the details of a single matter that one desires, the will is expressed equally in each of them; however, whether it's different matters that one desires, the will is not expressed differently according to the matter desired.
This characteristic of the will is expressed in all divine precepts (since these ARE the Divine Will) and, primarily, in the precept of purity and impurity.
For example, a man becomes completely impure by merely touching an impure object; that is, it makes no difference how close he was to the impurity, even the slightest touch results in total impurity. This is because the precepts of purity and impurity contain the revealed Divine Will, therefore there is no difference between much and little.
This concept applies to our daily service to God: Our sages say (Pirkei Avot 2:1) that one should be careful in fulfilling both the most important and the least important commandments. Although one might ask, "How can one be careful with commandments of great importance in the same way as with commandments of little importance? Neglecting something of little importance is only a minimal connection to evil; how can it be compared to neglecting something important?" Moreover, the Torah itself established this distinction between "very important commandments" and "less important commandments"!
Here comes the teaching of the precepts of purity and impurity: from the point of view of reason and logic in the Divine precepts (mitzvot), there are differences, some more important and others less important, but from the point of view of the essence of the Divine precepts, which is the Divine Will, they are all equal.
Extracted from Likutei Sichot, Volume 13, page 66