Hence, everything whose prohibition has not been specified by Allah or His Messenger—whether it concerns food, drink, or clothing—may not be declared unlawful; for Allah has already detailed for us what He has forbidden. Whatever is truly prohibited must therefore be set out explicitly. Just as it is impermissible to permit what Allah has forbidden, it is equally impermissible to forbid what Allah has overlooked and not prohibited.
The governing rule is this: every pure food that causes no harm is lawful. Impure food, by contrast—such as carrion, blood, dung, urine, wine, cannabis, and anything rendered impure through contact with filth—is forbidden because it is foul and harmful. The Exalted says, “Forbidden to you are carrion, blood, and the flesh of swine …” (Q. 5:3).
Carrion is any animal whose life has ended without lawful slaughter. It is prohibited because it provides foul nourishment, and the nourisher resembles the thing from which he is nourished. One of the beauties of the Sharia is its prohibition of such things. Yet if a person is compelled by necessity to eat it, it becomes lawful for him, and the aspect of filth is lifted in that state of necessity: the substance does not exist independently within the body but results from an interaction between recipient and agent. Necessity prevents the body from assimilating the impurity contained in the food, so the resulting harm does not arise. That harm is contingent on deliberate choice, through which the body accepts the impurity of the food; when choice is removed, the condition for acceptance is removed and the harm does not occur at all.
As for blood, the reference is to the blood that flows freely. The people of the pre-Islamic era would collect it in vessels, roast it, and eat it. What remains in the crevices of the meat after slaughter, or in the veins, is permissible; even if it comes onto one’s hand or a piece of cotton, it does not render them impure.
Shaykh al-Islam Ibn Taymiyyah (d. 728 AH) said, “The correct view is that only the blood which …”