Sefer HaChinuch ספר החינוך

The commandment of the matter of the impurity of semen, which is impure and renders impure: That semen be impure and render impure, as it is stated (Leviticus 15:16), “And if semen come out from a man.”

It is from the roots of the commandment [that] since this matter only happens from the angle of thoughts of physical desires, the perfect Torah informed us that the body is called impure with them. As the essence of his being in the world is only to understand ideas and to serve his Creator. Hence when that thing that shows about him [that he is] leaning to physical desire, happens to him; it is fitting to stop in his impurity for a day, in order that he cleans his thoughts properly, and then become pure.

From the laws of the commandment is that which they, may their memory be blessed, said (Niddah 43b) that the measure for one who touches [it] is like [the size of] a lentil, and for one who [experiences] the emission [it] is with the smallest amount. And it is one whether he [experiences] the emission accidentally or volitionally; he is impure. And the seed of a young child does not render impure. And red seed does not render impure, but rather [only] white. And any seed that a man does not feel — not at the beginning and not at the end — is not impure. And one who has [improper] thoughts at night, and saw in a dream that he had sexual relations and got up and found his flesh [to be] warm, is impure — even though he did not [knowingly experience] semen. And the rest of its details are elucidated in Tractate Zavim. (See Mishneh Torah, Laws of Offerings for Those with Incomplete Atonement 1.)

And the impurity of semen is practiced in every place and at all times. But today that — due to our sins — we have neither a Temple nor consecrated things, we do not need to be careful about the matter of impurity. And nonetheless, Ezra and his court ordained immersion for those with an emission, so that they be cleaner and purer in their thoughts, and that men not be found with their wives [constantly] like chickens. And in his time, no man would pray or study Torah until after immersion. But at this time, they ruled in the Gemara (Berakhot 22a) that the Sages rescinded [the law of this] immersion. And from [then], no man prevents himself from praying or studying — also placing tefillin — because of an emission. And they also rescinded taking [of waters] — which is rinsing the body with nine kav of water. And [so] now, they do not immerse, nor rinse [off] at all. And nonetheless, a soulful person who purifies himself from his emission also today [is exhibiting] a good and praiseworthy trait, and a blessing will come to him. And one who knows the way of purity and much good should hold on to it.