Where do you draw the line?
Hard boundary cases don't dissolve clear ones. We can't say exactly when dusk becomes night and can still tell noon from midnight.
If eating animals is wrong because they can suffer, the same reasoning reaches shrimp, oysters, insects and perhaps beyond, and nobody can say where suffering actually stops. A moral rule whose boundary no one can locate looks arbitrary, and drawing it at vertebrates or at mammals seems to be a matter of sentiment rather than principle. Without a principled line, the demand to stop eating animals has no clear content, and it is unclear why anyone should be bound by it.
The question is a fair one and the answer is genuinely uncertain at the edges. Insect sentience is unresolved; bivalves are debated; the evidence for cephalopods and decapods was strong enough to change UK law in 2022.
Uncertainty at a boundary does not propagate to the centre. This is the sorites problem, and it applies to most useful categories (baldness, adulthood, poverty) without anyone concluding that the categories are empty. Pigs, cattle and chickens are not borderline: they have nervous systems, nociception, and behaviour consistent with pain and distress, and they account for the overwhelming majority of animals farmed. One can act on the clear cases while the hard ones remain open.
Rejecting a workable option because it falls short of a perfect one. Also called the perfect solution fallacy. The relevant comparison is with the realistic alternative, not with perfection.
How it applies here: Demands a perfectly precise criterion before any distinction may be acted on, a standard that would disqualify most working categories.
Claiming that one step must lead to an extreme outcome without showing why each step in the chain follows from the one before it.
How it applies here: Implies that admitting one case commits you to every case down to insects and bacteria, without showing why each step follows from the one before it.