Halakhah for Niddah 50:37

And we have already said in the warning of the 'torn' animal (Sefer HaChinukh 73) and in the prohibitions of foods (Sefer HaChinukh 147, 148) that God, blessed be He, distanced His people that He chose from all the things that hurt the body, in that it is a vessel for the soul that knows its Creator. And so too from this root did He distance us from the woman who has given birth, from the menstruant and from the zavah (a woman with an irregular discharge) for a few days, until they become cleansed from that excess that they have, which is a bad and infectious thing. Therefore they, may their memory be blessed, said (Rashi on Leviticus 12:2) that a woman never [experiences] blood unless her head and limbs are heavy upon her. And there is no doubt that this excess makes them, and anyone who comes close to them at all, sick. And all the more so with sexual relations, which is the epitome of physical closeness. And also the offspring that is born [from a union] while that excess overpowers her is destined for bad illnesses. And from this root that we said from the angle of excess, the Torah commanded that she be impure from the male seven [days] and two weeks from the female. As there is no doubt that the excesses of the woman that gives birth to a female are greater than one who gives birth to a male - since conception of a male indicates heat in the female. And [it is] like they, may their memory be blessed, said (Berakhot 60a), "'If a woman conceives (literally, gives seed) and gives birth to a male - [if] a woman gives seed first, she gives birth to a male." And it is well-known that the excesses of heat are few and its action is always quick - it is a logical thing. And therefore seven days of cleansing suffices with the pregnancy of a male.

The root of the commandment is revealed in the verse: It is in order that we always remember all of the commandments of God. And there is no better reminder in the world than carrying the seal of the Master on the clothes that one wears at all times, as a person is [always attentive] to his clothes. And this is what is stated in the verse (Numbers 15:39), "and you will recall all the commandments of the Lord." And they, may their memory be blessed, said (Midrash Tanchuma, Korach 12) that the word 'tsitsit' alludes to the six hundred and thirteen commandments (in the numerical equivalent of the letters) when combined with the eight strings of the fringes and their five knots. And my heart also tells me that there is a reminder and allusion here that the soul and body of man all belong to God, Blessed be He. As the white portion corresponds to the body which is from the land, which was made from the snow, which is white, as we find in Pirkei D'Rabbi Eliezer 3, "From where is the land from? From the snow that is under the Holy Throne.” And the threads [also] allude to the body, as the matter that they said, that the initial formation of a body is like threads. [It is] as they, may their memory be blessed, said (Niddah 25b), "Rabbi Amram said, 'The two thighs are like two strands of crimson, the two forearms are like two strands of crimson.'" The blue (tekhelet), the appearance of which, is like the appearance of the sky hints to the soul which is from the upper beings. And they hinted to this in their saying (Menachot 47b), "What [makes] tekhelet different than all other colors? Because tekhelet is like the sea, and the sea is like the sky, and the sky is like the Throne of Glory, as it is stated (Exodus 24:10), 'And they saw the God of Israel, etc.' and it states (Ezekiel 1:26), 'The Throne appeared as sapphire stone'" - and the souls of the righteous are stored underneath the Throne. And because of this, they said (Menachot 39a) that we wrap the string of tekhelet around the white, as the soul is above and the body below. They said that we make seven or thirteen windings [around the white strings] to allude to the heavens and the divisions between them. And it is as they said (Menachot 39a), "It is taught, 'One who wishes to do fewer should not do fewer than seven, nor should he add more than thirteen, corresponding to the seven heavens and the six air-spaces between them.'"