 Consider the argument Penguin \(\Rightarrow\) Bird \(\rightarrow\) has wings. Since penguins do not fly, we know that penguins are exceptional birds, at least with respect to the property of flying. A very cautious reasoner may take this to be a reason to be skeptical about attributing other typical properties of birds to penguins. NMLs that do not allow to derive has wings are said to suffer from the Drowning Problem   (Benferhat et al., 1993). 