Among the most well-known precepts of Judaism – to Gentiles as well as to Jews -- are the laws of Kashrut. The Torah introduces this vast topic by
classifying living things.

First, mammals:
And Hashem spoke to Moshe and to Aharon, saying to them, Speak to the Children of Israel, saying: These are the living things that you shall eat (TOCHELU) from among all the animals that are upon the earth. Any
animal that has completely cloven hooves and chews the cud, it shall you eat (OTAH TOCHEILU). …Of the flesh [of creatures without both signs] you shall not eat (LO TOCHEILU) … (Vayikra 11:1-3, 8).

Then aquatic creatures:
This you shall eat (TOCHELU) from all that is in the
water: everything that has fins and scales in the water, in seas or rivers, those you shall eat (OTAM TOCHEILU). But anything that does not have fins and scales in the seas or rivers, …of their flesh you shall not eat
(LO TOCHEILU)…(verses 9-11).

Next are birds (verses 13-19) and insects (verses 20-23). After discussing other laws involving living things (verses 24-44), the passage concludes:
For I am Hashem Who brought you up from the land of Egypt to be for you
G-d; you shall be holy, for I am holy. This is the teaching of the animals, and the birds, and every living thing that swarms in the waters, and of every creature that multiplies on the earth. To distinguish (L’HAVDIL) between the unclean and the clean, and between the animals that are eaten and the animals that shall not be eaten (verses 45-47).

These are examples of what might be called “optional mitzvot”, since there is no requirement to eat meat. However, one who wants to eat meat may partake only of permitted species.

Rashi, based on the Torat Kohanim (3:1-2) here, notes the way the Torah speaks of these creatures in both positive and negative terms, which seems redundant:
“it shall you eat (OTAH TOCHEILU): [Only it,] and not an unclean animal. – But, it has already been prohibited [by the words you shall not eat
(LO TOCHEILU)]! -- However, this (verse) means that one transgresses thereby both a positive and negative commandment.”
One would be violating you shall not eat as well as avoiding the obligation it shall you eat.

But, how? Ramban (Commentary to Rambam’s Sefer HaMitzvot, Introduction, Root 6) does not include it shall you eat among the commandments. Sefer HaChinuch (ascribed to either R. Aharon HaLevi or R. Pinchas HaLevi of Barcelona, mid-13th Century; Commandments § 153-158, 162-165) agrees with Ramban’s reasoning:
“Since the Torah prohibited some animals to us, He would be obliged in any event to inform us of the distinguishing marks of the clean ones, in order to separate us from what is forbidden; and it is thus not fit at all to be counted as a commandment in the reckoning.”

However, the Chinuch committed himself at the outset to follow Rambam’s arrangement of mitzvot. And Rambam (Sefer HaMitzvot, Positive Commandments § 149-152, Negative Commandment § 172-179; Mishneh Torah “Introduction and Laws of Forbidden Foods” 1:1; 2:1-4) states unequivocally that there are four positive
commandments here, “to examine” creatures for the marks of cleanness. Accordingly, the Chinuch defines them:
§ 153 to examine the marks of a domestic or wild
animal (see also 20:25)
§ 155 to examine the marks of fish
§ 158 to examine the marks of locusts
§ 470 to examine the marks of birds (based on Devarim 14:11).
The practical consequence of ignoring these positive
commandments is,
“If someone violated this and made no examination, but merely saw one distinguishing mark and relied on it to eat it – even if he should discover afterward that he ate permissible food, he would thus disobey this positive commandment of examining the
distinguishing marks.”

Rambam cites the concluding verse as his source:
(L’HAVDIL) between the unclean and the clean …
Torat Kohanim (6) says on this:
“Not only one who has learned, but rather one who knows which species are clean and which are unclean.”

Malbim (R. Meir Leib ben Yechiel Michael, 1809-1877)
explains the difference between “learning” and “knowing”: Had the Torah required us “to learn,” then mere theoretical knowledge would have sufficed; rather, we are obligated L’HAVDIL -- To distinguish – to utilize sense perception to identify kosher creatures.

In the view of some authorities (e.g., Responsa Radbaz [R. David ben Shlomo ibn Avi Zimra, 1479-1573],VIII chap. 1; Aruch HaShulchan [R. Yechiel Michel Epstein, 1829-1908]; Yoreh Deiah 79:1-2), the obligation to examine, according to Rambam, occurs only if one wants to eat.

At the beginning of the “Laws of Forbidden Foods” however, Rambam formulates the mitzvah:
“There is a positive commandment to know the signs that distinguish … [emphasis added]”

Tzafnat Pa’neiach (R. Yosef Rozin, the “Rogatchover Gaon,” 1858-1936) on the Rambam (loc. cit.) and on Vayikra, p. 119f. puts forward the thesis that it is the signs themselves that make these creatures kosher. He cites, among other sources, Bava Batra 16a, where Iyov declares before G-d,
“You created the ox whose hooves are cloven, You created the donkey whose hooves are closed.”

Consequently, the actual knowing of the signs is a positive command, independent of their usefulness in avoiding the eating of forbidden meat. [Thus, we should all know kosher creatures firsthand, even if we don’t eat all of them; and this can be fulfilled by vegetarians as well! The one-day conference on “The Mesorah of Kosher Birds and Animals” sponsored by the UOJCA on May 2, 2004 enabled many to fulfill these four positive mitzvot.]

This knowing connects on the deepest level with the special sanctity of the Jewish people. Even the saintly Noach needed to learn, from the Torah of Israel, which animals were tahor, (see Rashi on Bereishit 7:2). And the saintly Iyov as well associates the knowledge of these signs with Hashem’s ordering of Creation.

Noach and Iyov learned about it. But only Israel knows and lives it.