LOGIC. 81 



accidents, or fallacia accidentis, consists in deriving general con- 

 clusions from data which are only accidentally or temporarily true. 

 Lord Bacon, in his Novum Organum, arranged the various sources 

 of error in opinion, under four heads ; which he fancifully called, 

 idola tribus, or general errors of parties and sects ; idola specus, or 

 peculiar errors of individuals ; idola fori, or errors of language and 

 conception ; and idola theatri, or errors of perception and specu- 

 lation. 



5. Under the head of Grounds of Judgment, we would treat 

 of the evidence, or proof, on which our premises rest ; and the de- 

 gree of weight, or credence, to be given to them. Premises, we 

 have said, are results of judgment; and a judgment we would define 

 to be an act of the mind, by which it perceives the correspondence, 

 or the disagreement of two ideas. The ground, or basis, on which a 

 judgment rests, is called Evidence ; which may be either personal or 

 historical, factive or deductive. Personal evidence, is that afforded 

 by our own senses or reasoning powers ; being the result of con- 

 sciousness, or perception ; or at least founded thereon ; as in experi- 

 ments, observations, and axioms, which we perceive, feel, or recog- 

 nise to be true. Historical evidence, is that which we have on the 

 authority of others, as the testimony of witnesses, and the statements 

 of writers. Factive evidence is that which rests immediately on 

 perception or consciousness, or on the memory of past perception or 

 consciousness ; while deductive evidence is that which is deduced 

 from factive, by some process of reasoning ; and which is used for 

 premises in deriving farther conclusions. 



As regards its degree of certainty, evidence is either indubitable 

 or probable. Demonstrative evidence, such as is employed in the 

 exact sciences, resting on definitions or axioms, is usually considered 

 indubitable ; and distinguished as mathematical certainty. Percep- 

 tive evidence, derived from experiments or observations, ranks nearly 

 as high, and is termed physical certainty. Historical evidence,, 

 strongly corroborated, is next to indubitable, and is characterized as 

 moral certainty. Analogical, or inductive evidence, founded on the 

 presumption of similar effects from similar causes, or of like condi- 

 tions from like circumstances, has various degrees of probability, in 

 different cases, and requires to be carefully weighed. Such evidence, 

 admits, in some cases, of mathematical valuation ; and some estimate 

 of its value we are often compelled to make, in the ordinary con- 

 cerns of life. The Calculus of probabilities, in which such valua- 

 tion is most accurately made, belongs to the department of Mathe- 

 matics ; and is the basis of life insurance, and other important 

 operations. 



6. The Uses of Reason are not confined to the deriving of con- 

 clusions, nor even to the verification of premises : but they extend 

 also to the invention of new premises, or the discovery of new 

 truths ; and to the methods of prosecuting such discoveries. New 

 truths are most frequently suggested to the mind, by analogy, or by 

 induction. Thus, when it became known that lead was fusible, it 

 was suggested by analogy that other metals might also be melted ; 

 and when this was found to be the case with all the metals then 

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