An inference having been made, it comes up for reconsideration. This will take two directions
First, inquiry into the grounds of the premises.
Second, into the validity of the reasoning.
The latter consists in supplying as premises what were "leading principles".
Both processes according to me may be carried back indefinitely, without reaching any end.
The proof that the first may be carried back indefinitely is this:
Every premise is a proposition; no proposition is directly observed; whatever is not directly observed is the conclusion of an inference or at least it has to be justified by a proof with premises.
The real mental process of reaching a proposition isn’t an infinite series of inferences but that is its equivalent when analyzed.
Like an infinite series in algebra.