The Jewish dietary laws and customs are complex, having developed over the course of many centuries and have been interpreted and applied differently in different communities around the world. However, the underlying foundation is the same everywhere, even if the manner in which different Jews observe the customs differs. I present here a broad overview and explanation.

Jewish dietary laws derive from several sources and have developed over time. The Bible contains a number of seemingly unrelated restrictions which form the basis of the laws of Kashrut:

• Permitted and forbidden animals are listed in the Book of Leviticus. No reasons are given, though generalizations can be drawn after-the-fact. The notion of obedience to God's will is sufficient reason for the Torah. By and large, permitted animals are domesticated animals which chew the cud and have cloven hoofs.
• Fowl are permitted.
• Permitted birds have a crop, sac in the gizzard that can be peeled off, a fourth front toe and another toe in the back, and are not birds of prey.
• Fish are permitted if they have fins and scales at some point during their life cycle. Sea creatures which lack fins and scales, such as sharks, are not permitted. Shellfish (including lobster, shrimp, clams, and oysters are not permitted.
• Insects and reptiles are permitted or forbidden on a case-by-case basis. Generally speaking, permitted creatures have four legs, four wings, leaping legs and wings covering the greater part of the body.
• Blood, the life force of the animal, may not be eaten out of respect for the life that is taken when an animal is slaughtered for food. The blood must be completely removed; this is done by salting.
• A kid may not be boiled in its mother's milk. The boiling of a kid in its mother's milk was an ancient Canaanite fertility ritual which compounded the symbols of the baby kid with the mother's milk. This was considered an offensive and idolatrous ritual to the ancient Israelites and is forbidden by the Torah. The Torah emphasizes the importance of avoiding pagan idolatrous customs and it is clear that this ritual was considered especially offensive. The prohibition of boiling a kid in its mother's milk forms the basis for the later expansion of kashrut laws to exclude the mixing of milk and meat products.
• The sciatic nerve is forbidden on the basis of the account in the Book of Genesis in which Jacob wrestles with the angel (Genesis 32:33).

The rabbis expanded the the dietary restrictions to safeguard against transgressing the prohibitions of the Torah. Their legislations includes:

• Separation of milk and meal products to prevent inadvertent transgression of the prohibition against boiling a kid in its mother's milk. Since one cannot always be sure of the source of the meat and the milk, they developed a system of cooking and eating that required completely separation of cooking and eating utensils, and forbid serving milk and meat products on the same table at the same time.
• The rabbis further regulated the laws of Shechitah (kosher slaughtering) with respect to both the procedures and the knife employed.
• The parts of animal which could be eaten and condition of the animal at the time of Shechitah were further delineated.
• Controversies about the status of various cheeses and wines was discussed at length.
• Length of time between eating milk and meat

Later discussion include the controversies about the status of various cheeses and wines and the status of glass dishes, which are used by some for both milk and meat dishes, because they are non-porous and therefore cannot absorb food particles. To encapsulate the argument concerning glass dishes: While it is possible to clean food off glass dishes completely, one loses the visual distinction between milk and meat utensils and the pedagogic value of having separate dishes.

Today, addition concerns have arisen that are of an ethical nature. The cruel environment in which veal is produced has given rise to the notion that on moral grounds, veal can be considered to be non-kosher. Judaism teaches the principle of tza'ar baalei chayim (concern for the suffering of animals) and the manner in which veal is raised is a clear violation of this principle. Similarly, table grapes grown in the United States have come under scrutiny and been declared unkosher by some groups on the basis of the danger their growing and harvesting present to the migrant farm-workers who pick them and are subjected to dangerous chemicals.

The opening chapters of the Torah make clear that God's intention for humanity was a vegetarian diet. Adam and Eve are permitted the fruits and vegetables of the Garden of Eden, but not animals. It is only after the Flood that God permits people to consume flesh and here it appears to be a compromise, at best. Hence, some Jews today are calling for a vegetarian diet as the purest form of Jewish diet.

Today, there is a wide spectrum of ways in which Jews observe the laws of kashrut. Some keep a strict vegetarian diet. Others observe what they term "biblical kashrut" which means that they observe the restrictions set out in the Torah, but not the enlargement of rabbinic legislation. This means that they avoid forbidden foods and the blood of animals. Others separate milk and meal to varying degrees; some keep separate dishes, pots, and utensils; others cook and store milk and meat separately, but keep only one set of dishes, pots, and utensils. Some people keep a kosher kitchen at home, and eat vegetarian food outside the house. Others maintain a kosher kitchen at home only. There is great variety in the way people observe the laws of kashrut in their lives.

I am frequently asked: What is the point of keeping kosher? Several decades ago it was popular to say that kashrut was a Jewish health regimen, but this is not the case. We know that proper handling and cooking of pork and shellfish render them harmless for consumption. Indeed, it would appear that health is not the issue at all. It is my opinion that there are two other principles operating behind the scenes here: (1) Kashrut requires obedience to a set of practices and standards that are NOT rational; they simply are. That means that one keeps kosher because it is a Jewish obligation. No more, no less. It is a purely religious act and, as such, has great spiritual value. What is more, it is an identifying feature in one's life. (2) When we consider what aspects of being human are most like other animals, eating and procreating float to the top of the list. Judaism bids us elevate these animal functions from the level of the mundane to the level of the kedushah (holiness). In both cases, there is a Jewish way to approach these baser functions in order to elevate them to holy acts. In the case of eating, carefully considering the source of what we eat -- and in the case of meat, that a life was sacrificed for our nutrition and eating pleasure -- as well as the means of preparation, is of great value. If something so mundane as eating can become an act of kedushah, then surely all our lives can be elevated to this level if we but give our words and deeds the attention they deserve.