For the source text click/tap here: Chullin 110
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The Liver as a Special Halakhic Category
The laws of liver are unusually revealing because they demonstrate that halakhah does not simply attach a prohibition to a chemically identifiable substance called “blood.” Rather, it distinguishes between blood according to its location, state, movement, and halakhic history. The same material may be permitted while incorporated within one structure and prohibited after it emerges from that structure.
“It is all blood, yet the Torah permitted it”
That is quite different from ordinary meat. With ordinary meat we tend to say that blood contained in the tissue has not yet acquired the operative status of prohibited dam; once it separates—* פירש*—the prohibition takes effect. Liver presents almost the reverse phenomenon: according to Rabbeinu Tam and the authorities following his conceptual direction, what is inside the liver was already identifiable as blood, yet the Torah specifically permitted it. Consequently, its subsequent emergence creates the difficult question: how can something that was permitted as blood suddenly become forbidden simply because it moved?