Issi ben Guria demonstrated that eating meat cooked in milk is forbidden through a verbal comparison between two passages. The word "holiness" appears in (Deuteronomy 14:21), where the meat-and-milk prohibition is stated, and also in (Exodus 22:30): "And men of holiness shall you be unto Me, and flesh in the field, torn, you shall not eat."
In the Exodus passage, "holiness" is connected to a prohibition against eating — specifically, eating torn flesh (treifah). In the Deuteronomy passage, "holiness" appears in the context of the meat-and-milk prohibition. Since both verses invoke "holiness," the Mekhilta transfers the eating prohibition from one to the other.
Just as the "holiness" verse in Exodus forbids eating, the "holiness" verse in Deuteronomy also forbids eating. The verbal link establishes that the meat-and-milk prohibition includes a ban on consumption, not merely on cooking.
This is the third independent derivation of the eating prohibition — alongside the arguments from the Passover offering and the thigh sinew. The Mekhilta assembles multiple proofs for the same conclusion, each using a different legal technique: a fortiori reasoning, verbal analogy, and textual juxtaposition. The convergence of all three methods on a single conclusion demonstrates that the eating prohibition is firmly embedded in the Torah's structure, even though no single verse states it directly.