The arguer in this example states that cycling on the grass is prohibited and supports this claim by saying that walking on the grass is prohibited. 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 ‘cycling on the grass’, X by ‘is prohibited’, and b by ‘walking on the grass’.
Cycling on the grass (a) is prohibited (X), because walking on the grass (b) is prohibited (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 value (legal qualifications), which means that we are dealing with a first-order subject argument supporting a value with another value (1 pre VV).
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 cycling on the grass is analogous to walking on the grass. We can therefore call such an argument an ‘argument from analogy’.
The example is from R. Kolb (2016), The law of treaties: An introduction (p. 152). Cheltenham: Edward Elgar Publishing.