Rule #3: We should consider whether a link between S and P is
plausible in light of other knowledge we possess.
To see the point of this rule, consider two contrasting cases.
If chemists discovered a new metal, they would determine its
melting point in a laboratory, and a single test would be
sufficient to support the generalization that all instances of
the metal would melt at that temperature. Our theoretical
knowledge about physical substances tells us that the melting
point of a metal is fixed by its atomic structure, which defines
that type of metal.
On the other hand, the generalization that all swans are white
was confirmed by countless instances over a long period of time.
Yet, this generalization was regarded as somewhat shaky, because
biological theory gives us no reason for thinking that all swans
must be the same color. Eventually, black swans were indeed
discovered in Australia.