The argument from similarity is a first-order subject argument that supports a fact with another fact (1 sub FF).
ANALYSIS OF AN EXAMPLE
This ad links gun control to the holocaust. As part of the argumentation in support of the view that the Second Amendment is a good thing, the arguer states that “In 1939, Germany established gun control. From 1939 to 1945, six million Jews and seven million others, unable to defend themselves, were exterminated”. We can reconstruct the argument as follows:
Establishing gun control will lead to genocide, because in Germany it led to genocide
The arguer states in the conclusion that gun control will have the same consequences in the U.S. now as it did in Germany back then. Since the argument fits the form a is X, because b is X, it can be identified as a first-order subject argument. In this specific case, a is instantiated by ‘in the present-day U.S.’, X by ‘establishing gun control (will) le(a)d to genocide’, and b by ‘in historical Germany’.
In the present-day U.S. (a) establishing gun control will lead to genocide (X), because in historical Germany (b) establishing gun control led to genocide (X)
First-order subject arguments are further differentiated by identifying the types of statement in the conclusion and the premise. In this case, both are statements of fact (one future and one past), which means that we are dealing with a first-order subject argument supporting a fact with another fact (1 pre FF).
The trivial name of first-order subject arguments is derived from the characterization of the relationship between the subjects a and b. In this example, the argument draws on the idea that when it comes to the effects of establishing gun control, present-day U.S. is similar to historical Germany. We can therefore call such an argument an ‘argument from similarity’.