Ramban - Leviticus.19.2
קדושים תהיו ולפי דעתי אין הפרישות הזו לפרוש מן העריות כדברי הרב אבל הפרישות היא המוזכרת בכל מקום בתלמוד שבעליה נקראים פרושים והענין כי התורה הזהירה בעריות ובמאכלים האסורים והתירה הביאה איש באשתו ואכילת הבשר והיין א"כ ימצא בעל התאוה מקום להיות שטוף בזמת אשתו או נשיו הרבות ולהיות בסובאי יין בזוללי בשר למו וידבר כרצונו בכל הנבלות שלא הוזכר איסור זה בתורה והנה יהיה נבל ברשות התורה לפיכך בא הכתוב אחרי שפרט האיסורים שאסר אותם לגמרי וצוה בדבר כללי שנהיה פרושים מן המותרות ימעט במשגל כענין שאמרו (ברכות כב) שלא יהיו תלמידי חכמים מצויין אצל נשותיהן כתרנגולין ולא ישמש אלא כפי הצריך בקיום המצוה ממנו ויקדש עצמו מן היין במיעוטו כמו שקרא הכתוב (במדבר ו ה) הנזיר קדוש ויזכור הרעות הנזכרות ממנו בתורה (בראשית ט כא) בנח ובלוט וכן יפריש עצמו מן הטומאה אע"פ שלא הוזהרנו ממנה בתורה כמו שהזכירו (חגיגה יח) בגדי עם הארץ מדרס לפרושים וכמו שנקרא הנזיר קדוש (במדבר ו ח) בשמרו מטומאת המת גם כן וגם ישמור פיו ולשונו מהתגאל ברבוי האכילה הגסה ומן הדבור הנמאס כענין שהזכיר הכתוב (ישעיהו ט טז) וכל פה דובר נבלה ויקדש עצמו בזה עד שיגיע לפרישות כמה שאמרו על רבי חייא שלא שח שיחה בטלה מימיו באלו ובכיוצא בהן באה המצוה הזאת הכללית אחרי שפרט כל העבירות שהן אסורות לגמרי עד שיכנס בכלל זאת הצוואה הנקיות בידיו וגופו כי אע"פ שאלו מצות מדבריהם עיקר הכתוב בכיוצא בזה יזהיר שנהיה נקיים וטהורים ופרושים מהמון בני אדם שהם מלכלכים עצמם במותרות ובכיעורים וזה דרך התורה לפרוט ולכלול בכיוצא בזה כי אחרי אזהרת פרטי הדינין בכל משא ומתן שבין בני אדם לא תגנוב ולא תגזול ולא תונו ושאר האזהרות אמר בכלל ועשית הישר והטוב (דברים ו יח) שיכניס בעשה היושר וההשויה וכל לפנים משורת הדין לרצון חבריו כאשר אפרש (שם) בהגיעי למקומו ברצון הקב"ה וכן בענין השבת אסר המלאכות בלאו והטרחים בעשה כללי שנאמר תשבות ועוד אפרש זה (רמב"ן על ויקרא כ״ג:כ״ד) בע"ה:
Translation:
YE SHALL BE HOLY. “Abstain from the forbidden sexual relationships [mentioned in the preceding section] and from [other] sin, because wherever you find [in the Torah] a warning to guard against immorality, you find the mention of ‘holiness.’” This is Rashi’s language.
But in the Torath Kohanim, I have seen it mentioned without any qualification [i.e., without any particular reference to immorality, as Rashi expressed it], saying: “Be self-restraining.” Similarly, the Rabbis taught there: “And ye shall sanctify yourselves, and be ye holy, for I am Holy. Just as I am Holy, so be you holy. Just as I am Pure, so be you pure.”
In my opinion, this abstinence does not refer only to restraint from acts of immorality, as the Rabbi [Rashi] wrote. Still, it is rather the self-control mentioned throughout the Talmud, which confers upon those who practice it the name of P’rushim, [literally: “those who are separated” from self-indulgence, as will be explained, or those who practice self-restraint]. The meaning thereof is as follows: The Torah has admonished us against immorality and forbidden foods but permitted sexual intercourse between man and his wife and the eating of [certain] meat and wine. If so, a man of desire could consider this to be a permission to be passionately addicted to sexual intercourse with his wife or many wives, and be among winebibbers, among gluttonous eaters of flesh, and speak all profanities freely, since this prohibition has not been [expressly] mentioned in the Torah. Thus, he will become a sordid person within the permissible realm of the Torah!
Therefore, after having listed the matters which He prohibited altogether, Scripture followed them up with a general command that we practice moderation even on the issues which are permitted, [such as in the following]: One should minimize sexual intercourse, similar to that which the Rabbis have said, “So that the disciples of the Sages should not be found together with their wives as often as the hens,” and he should not engage in it except as required in fulfillment of the commandment thereof. He should also sanctify himself [to self-restraint] by using wine in small amounts, just as Scripture calls a Nazirite “holy” [for abstaining from wine and strong drink], and he should remember the evils which the Torah mentioned as following from [drinking wine] in the cases of Noah and Lot.
Similarly, he should keep himself away from impurity [in his ordinary daily activity], even though we have not been admonished against it in the Torah, similar to that which the Rabbis have said: “For the P’rushim, the clothes of the unlearned are considered as if trodden upon by a zav” [or zavah — a man or woman has suffered a flux], and just as the Nazirite is called “holy” because of guarding himself from the impurity of the dead.
Likewise, he should guard his mouth and tongue against being defiled by excessive food and lewd talk, similar to what Scripture states. Every mouth speaketh wantonness, and he should purify himself in this respect until he reaches the degree known as [complete] “self-restraint,” as the Rabbis said concerning Rabbi Chiya, that never in his life did he engage in unnecessary talk. It concerns these and similar matters that this general commandment [Ye shall be holy] is concerned after He had enumerated all individual deeds which are strictly forbidden so that cleanliness of hands and body are also included in this precept.
Scripture’s main intention is to warn us of such matters, that we should be [physically] clean and [ritually] pure and separated from the common people who soil themselves with luxuries and inappropriate things. Such is the way of the Torah, which, after it lists certain specific prohibitions, includes them all in a general precept.
Thus, after warning with detailed laws regarding all business dealings between people, such as not to steal or rob or to wrong one another, and other similar prohibitions, He said in general, And thou shalt do that which is right and good, thus including under a positive commandment the duty of doing that which is right and of agreeing to a compromise [when not to do so would be inequitable]; as well as all requirements to act “beyond” the line of justice [i.e., to be generous in not insisting upon one’s rights as defined by the strict letter of the law, but to agree to act “beyond” that line of the strict law] for the sake of pleasing one’s fellowman, as I will explain when I reach there [that verse], with the will of the Holy One, blessed be He.
Similarly, in the case of the Sabbath, He prohibited doing certain classes of work using a negative commandment and painstaking labors [not categorized as “work,” such as transferring heavy loads in one’s yard from one place to another, etc.] He included under a general positive commandment, as it is said, but on the seventh day, thou shalt rest. I will yet explain this with the help of G-d.