The Commens Dictionary
Quote from ‘On the Algebra of Logic’
The term logical leading principle we may take to mean the principle which must be supposed true in order to sustain the logical validity of any argument. Such a principle states that among all the states of things which can be supposed without conflict with logical principles, those in which the premiss of the argument would be true would also be cases of the truth of the conclusion. Nothing more than this would be relevant to the logical leading principle, which is, therefore, perfectly determinate and not vague, as we have seen an extralogical leading principle to be.