If someone decides to keep himself on a certain level of ritual purity and to guard against contamination, he is still considered definitely impure for the next level. This is because in his mind he decided only on the given level, and a conscious effort is required for the next one. Thus, the clothing of an ignorant person is considered impure for those who conduct themselves in accordance to the laws of ritual purity, but their clothing in turn is considered impure for those to eat the kohen's portion. Those who eat the kohen's portion (priests and their family) are considered impure for those who eat sacrifices.
Earlier we said that if no blood flowed during the shechitah, the meat does not become susceptible to ritual impurity. Rabbi Shimon disagrees and says that the shechitah alone prepares the meat to become impure. According to him, the reason that wetting the food makes it susceptible to impurity is because this is the final step in food preparation. But the final step in meat preparation is the shechitah itself. The shechitah then takes the place of wetting the food, and thus makes the meat susceptible to impurity.
Art: Christoffel Lubieniecki - Food Lovers