Passage Summary: Smith's cows are grazing on grass where half is moldy and makes them sick; the owner thinks the cows will figure it out and only eat the good grass because they can tell when something is bad for them.

Conclusion: Smith's cows will likely learn to eat only the grass that does not contain the sickness-inducing fungus.

Reasoning: Cows generally avoid eating patches of grass if they can sense a substance in it that has previously made them ill.

Analysis: The argument relies on a crucial missing link: the cows' ability to actually detect the fungus. If the fungus is invisible, odorless, and tasteless, the cows' natural avoidance mechanism won't do them any good. We are looking for a 'Necessary Assumption,' so ask yourself what must be true for this plan to work. If the cows can't distinguish the 'bad' grass from the 'good' grass before they take a bite, the conclusion that they will learn to graze 'exclusively' on the good grass falls apart. Apparently, Smith has a very high opinion of bovine sensory perception.