DEMONSTRATION AND SELF-EVIDENCE 17 



presupposed to any demonstration, even when they are not 

 explicitly expressed as premises, precisely because the knowl- 

 edge of proper concepts which is required for theses presupposes 

 and in a sense depends upon a prior grasp of common con- 

 cepts.^* Those which are logical in character function neces- 

 sarily as methodological principles which guarantee the integrity 

 of discourse without being built into it as doctrinal principles. 

 For example, the principle of contradiction is an absolutely 

 common methodological principle without which there could be 

 no discourse at all. No proposition can function properly as a 

 principle of demonstration except that it be firmly accepted 

 that the affirmation of its opposite is excluded in the face of its 

 own affirmation.^^ Of course axioms of an ontological character 

 (when illumined by the light of metaphysical abstraction) can 

 be used as premises in metaphysical discourse, just as axioms 

 of a logical character must be built into proofs in logical theory 

 as explicit premises. The reason for this is that metaphysics 

 and logic are common sciences, so that the principles common 

 to the other sciences are proper to them. As a matter of fact, 

 these common propositions can even be used as explicit pre- 

 mises in the particular sciences, though here they become 

 principles of dialectical rather than demonstrative discourse .^^ 



** Consider the relation of being to all other concepts. De Ver., q. 1, a. 1, resp.: 

 " Elud autem quod primo intellectus concipit quasi notissimum, et in quo omnes 

 conceptiones resolvit, est ens; " In III Met., lect. 5. Cf. Cajetan, Comm. In De 

 Ente et Essentia, q. 1. 



"^ In IV Met., lect. 6, n. 603: " Si igitur quis opinetur simul duo contradictoria 

 esse vera, opinando simul idem esse et non esse, habebit simul contrarias opiniones: 

 et ita contraria simul inerunt eidem, quod est impossibile. Non igitur contingit 

 aliquem circa haec interius mentiri et quod opinetur simul idem esse et non esse. 

 Et propter hoc omnes demonstrationes reducunt suas propositiones in hanc proposi- 

 tionem, sicut in ultimam opinionem omnibus communem: ipsa enim est naturaliter 

 principium et dignitas omnium dignitatum." Cf., also In I Post. Anal., lect. 6, n. 7. 



^' Though the direct use of logic is methodological, supplying either the rules of 

 demonstrative or dialectical discourse, logic can, along with metaphysics, because 

 of the correlatively common character of the formal subjects of each, supply 

 premises for argumentation in the particular sciences. Since demonstration requires 

 premises appropriate to the conclusion, the argumentation in some particular science 

 with a premise from metaphysics or logic will be dialectical at best. 



