154 GENERAL YIEW AND BASIS OF SCIENTIFIC RESEARCH. 



6 1st Criterion. The Principle of Contradiction. The 

 same attribute cannot at .the same time be affirmed and 

 denied of the same subject, or, the same subject cannot 

 have two contradictory attributes. 



' 2nd Criterion. The Principle of Identity. Concep- 

 tions which agree can be affirmed of the same subject at 

 the same time. This principle is the complement of the 

 former. 



' 3rd Criterion. The Principle of the Middle being 

 Excluded. Either a given judgment must be true or its 

 contradictory ; there is no middle course. 



' 4th Criterion. The Principle of Sufficient Reason. 

 Whatever exists, or is true, must have a sufficient reason 

 why the thing or proposition should be as it is, and not 

 otherwise. From this law are deduced such applications 

 as these : 1. Granting the reason, we must grant what 

 follows from it. On this depends syllogistic inference. 

 2. If we reject the consequent, we must reject the reason.' 

 ' The four criteria in question are useful in securing formal 

 truth,' 4 that is, in keeping our thoughts in harmony with 

 each other ; but for the discovery of material truth, for 

 giving us thoughts that are true representatives of facts, 

 they are either useless, or only useful as principles subor- 

 dinate to the higher criterion that every proposition 

 must rest on sufficient evidence.' c Viewed as instruments 

 for judging of material truth, they' (i.e. the criteria) 

 c sink into mere rules for the reception of evidence. The 

 first is a caution against receiving into our notion of a 

 subject any attribute that is irreconcileable with some 

 other, already proved upon evidence which we cannot doubt. 

 The second is a permission to receive attributes that are 

 not thus mutually opposed, or a hint to seek for such 

 only. The third would compel us to reconsider the 

 evidence of any proposition, when other evidence threatened 



