, we can proceed to assert
. There is always unavoidably something psychological about inference: inference is a method by which we arrive at new knowledge, and what is not psychological about it is the relation which allows us to infer correctly; but the actual passage from the assertion of
to the assertion of
is a psychological process, and we must not seek to represent it in purely logical terms.
In mathematical practice, when we infer, we have always some expression containing variable propositions, say