Mufaddal Ibn Umar is quoted in the books al-Kāfi and Amali to have asked Imam Sadiq (a.s.) why Allah (S.w.T.) has prohibited Carrion, blood and pork? Imam (a.s.) replied,
“Allah has not prohibited anything for men to take it Himself (we seek refuge in Allah (S.w.T.)). And He has not permitted things that He Himself didn’t like. (In other words Allah (S.w.T.) has not framed the rules of Harām and Halāl on the basis of His personal whims and fancies. The truth is that when Allah (S.w.T.) is Creator of everything, He alone knows what is good and necessary for the human body. Therefore he made it permissible due to His grace. He Alone knows what is harmful for the human body and, therefore prohibited it and made it Harām. But one who is helpless and there is no other way for survival is allowed to eat the same (prohibited) things. However, he must eat only as much as is necessary to save one’s life.”
After that he said, “Eating carrion makes ones body weak and thin. It destroys ones physical prowess and cuts off his generations. One who eats carrion dies a sudden death.”
It appears therefore that in the process of slaughtering, the blood gushes out and the meat is free of the impure blood with its toxic substances which flow in the veins. When an animal dies naturally or by any process where the blood remains in the body, the meat is contaminated with toxic substances, the consumption of which is injurious to health.
A heretic asked Imam Sadiq (a.s.) why Allah (S.w.T.) had prohibited carrion?
Imam (a.s.) said,
“Carrion is made Harām so that there should be difference between it and an animal upon which the name of Allah is recited. Also, the blood of a dead animal is not expelled from its body. It returns to its organs and makes its meat heavy and distasteful. Its meat is eaten with its blood.”
The heretic said, “Then the flesh of fish must also be carrion and Harām because blood is not expelled from it.”
Imam (a.s.) told him,
“The purification of fish is just that it should be removed from water and left outside to die by itself. It is not necessary to slaughter it because it does not possess (warm) blood. The same applies to locust.”
In the fourth volume of Bihār al-Anwār it is mentioned that the Imam also said that it is not necessary to slaughter fish because it doesn’t have much blood. And the blood that remains in its body is like the blood that remains in the body of an animal after it is slaughtered, which is harmless and Halāl.