the Law of Contagion holds that two things which have interacted, or were once part of a single entity, retain their connection and can exert influence over each other; the Law of Similarity holds that things which are similar or treated the same establish a connection and can affect each other.

Are there any relationships between two physical objects that are neither interactions nor similarities? If not, these two laws boil down two "things which are related can influence each other", which is a true statement. It is as reasonable to generalize this rule as it is to generalize any rule, until you have a more specific rule (e.g. "things which are capable of sending and receiving known types of waves to each other across the span separating them can influence each other") that does not generalize the same way.