 A further possibility for carving out the distinctive notion of necessity grounding logical consequence is the notion of apriority. Deductively valid arguments, whatever they are, can be known to be so without recourse to experience, so they must be knowable a priori. A constraint of apriority certainly seems to rule argument (3) out as deductively valid, and rightly so. However, it will not do to rule out argument (4). If we take arguments like (4) to turn not on matters of deductive validity but something else, such as an a priori knowable definition, then we must look elsewhere for a characterisation of logical consequence.