Condillac, who sought to reduce all reflection to sensation, and by his method the forerunner of the Idealism of Berkeley, of the Skepticism of Hume, of the Empiricism of the Scottish School, and of the Critical Philosophy of Kant.

Locke's chief work, An Essay concerning the Human Understanding, was first published in London, 1690. Since he would not admit that the conceptions arrived at by sense-perception are true pictures of the objects (figure in space may be objective; colours, sounds, &c. are not), he limited the truth of our thoughts to the objectively correct union and separation of the signs of things.' Connected with Locke are J. P. de Crousaz,2 Is. Watt,3 Condillac, and Hume." The Idealism of Berkeley (1685–1753), according to which only spirits and their ideas exist, since all unthinking objects are ideas of a percipient and thinking existence, and the Scottish School (Reid, Stewart, &c.), which returned to the assertion of innate activities as facts of inner experience, are, in spite of their polemic against it, essentially connected with the Lockian tendency.

6

§ 27. Leibniz (1646-1716) maintained against Locke the doctrine of innate ideas; but he explained every part of the contents of consciousness to be the production of the inner self-development of the mind (Seele). Leibniz found warrant for the objective truth of clear and distinct conceptions in a harmony between the soul

1 Essay, bk. iv. ch. v. § 2.

2 La Logique, Amst. 1712.

3 Logic, 1736.

4 Essai sur l'Origine des Connaissances humaines, 1745; Traité des Sensations, 1754; Logique, 1781.

5 Enquiry concerning the Human Understanding, 1748 [best edition by T. H. Green and T. H. Grosse, London, 1870].

6 [Best edition is the Collected Works of George Berkeley, D.D., Bishop of Cloyne, ed. by Prof. Fraser, Clarendon Press, 1871.]

E

Error arises
Dark and

and outer things pre-established by God. from a want of clearness and distinctness. confused knowledge may be raised by demonstration to clearness and distinctness. Leibniz (in opposition to Des Cartes) declared the logical rules to be criteria of truth not to be despised, because correctness of demonstration depended on their being followed. He held the principles of Contradiction and of Sufficient Reason to be the most general principles of all demonstration.

Taking his stand upon the Leibnizian theory, Wolff presented Logic (as he did all the philosophical disciplines) in systematic connection, according to mathematical method. He treated Logic as the doctrine of knowledge, and placed the logical forms in essential relation both to ontological forms and psychological laws.

The opinions of Leibniz upon the doctrine of knowledge are contained partly in small tracts, partly in his Nouveaux Essais sur l'Entendement humain, directed against Locke, and first published posthumously by Raspe, in 1765. Leibniz approved generally of the Cartesian principle, quicquid clare et distincte de re aliqua percipio, id est verum seu de ea enunciabile; but he held it necessary to prevent the frequent abuse of the principle by laying down criteria of clearness and distinctness. He, accordingly, defines the clear conception (notio clara) to be that which can recognise the object conceived, and distinguish it from others. The clear conception is either confused (confusa) or definite and distinct (distincta). Confusion is want of clearness in the particular attributes (notae). Distinctness or definiteness, on the other hand, is the clearness of the particular individual attributes which together make up the conception. In absolutely simple conceptions there is no distinction between clearness and distinctness. The distinct conception, finally, is adequate when the attributes of the attributes, on to the last simple elements,

are clearly conceived. These definitions are not in themselves free from fault. Distinctness and Confusion are specifically, not gradually, to be distinguished from Clearness and Unclearness, just as the accuracy and inaccuracy of a drawing are from the clearer and fainter outline. But the system of Pre-established Harmony cannot admit that error has a source specifically distinct from that of want of clearness. The possibility, which consists in freedom from inner contradiction, and becomes known by the complete resolution of conceptions into their component parts, is, according to Leibniz, the warrant of objective validity or truth. He says, in the above quoted tract: 'Patet etiam, quae tandem sit idea vera, quae falsa; vera scilicet quum notio est possibilis, falsa quum contradictionem involvit.' By the separation of a conception into its non-contradictory attributes, we recognise à priori its validity, but we recognise à posteriori its validity by experience. The truth of a proposition consists in its correspondence with the objects to which it refers. It is reached by accurate experience and correct logical proof. Meditationes (as above): De caetero non contemnenda veritatis enunciationum criteria sunt regulae communis Logicae, quibus etiam Geometrae utuntur, ut scilicet nihil admittatur pro certo, nisi accurata experientia vel firma demonstratione probatum; firma autem demonstratio est, quae praescriptam a Logica formam servat. For the principles of contradiction and sufficient reason as the grounds of all demonstration, see the Monadology (Principia Philosophiae), §§ 30-31. Leibniz wished to see a doctrine of probability added to Logic, as a second part.

Christian Wolff gave a systematic representation of Logic in his shorter German treatise-Vernünftige Gedanken von den Kräften des menschlichen Verstandes, 1710; and in his extensive work-Philosophia Rationalis sive Logica, 1728. He defined Logic to be scientiam dirigendi facultatem cognoscitivam in cognoscenda veritate. The rules, according to

2

1 See Leibnitii Meditationes de Cognitione, Veritate et Ideis [appended to Prof. Baynes' ed. of the Port-Royal Logic, 2nd ed. pp. 424-30]. 2 Log. Discursus Praeliminaris, § 61; Prolegomena, § 10.

which the human mind learns the essences of things, must on the one hand be psychological, and on the other ontological principles. It is advisable, because of its suitableness for Didactic, that Logic should precede Ontology and Psychology, and so Wolff makes it. The proof, however, of the logical axioms is not, therefore, to be omitted, but the more important doctrines of Ontology and Psychology must be presupposed in Logic, where they from the first vindicate their position, both by immediate evidence, and by their agreement with experience.3 Accordingly, Wolff places some psychological considerations, and a section de notitiis quibusdam generalibus entis,'5 at the head of his logical system. He divides Logic into theoretical and practical. The former treats of Notion, Judgment, and Inference; the latter, of the use of Logic in judging, and in the investigation of truth, in the study and composition of books, in the division of knowledge, in the comparative valuing of the individual powers of knowledge, and, lastly, in the practice of life, and in the study of Logic itself. Wolff gives as the nominal definition of truth-Est veritas consensus iudicii nostri cum obiecto seu re repraesentata;'6 and as its real definition- Veritas est determinabilitas praedicati per notionem subiecti.' The possible notion corresponds to the true affirmative judgment. Possibility consists in absence of contradiction." To this (Leibnizian) criterion Wolff refers the Cartesian, and also the criterion of conceivability given by Tschirnhausen (1651-1708), the contemporary of Leibniz, in his Medicina Mentis, 1687verum est quicquid concipi potest, falsum vero quod non concipi potest.'

10

Among the contemporaries of Leibniz, besides Tschirnhausen, Christian Thomasius (1655-1728) is to be mentioned,

1 Discursus Prael. § 89; Prolegom. § 28.

2 Discurs. Praclim. § 91: Methodum studendi praeferre maluimus methodo demonstrandi.'

3 Log. §§ 2, 28. 6 Ibid. § 505.

9 Ibid. § 518.

4 Ibid. § 30 ff.

7 Ibid. § 513.
10 Ibid. §§ 522, 528.

5 Ibid. § 59 ff.

8 Ibid. § 520.

3

§ 27. Leibniz and Wolff.

53

who sought to make Logic more practical, and believed that he had pointed out a middle way between the Aristotelian and Cartesian Logics. His special service (as Wolff's was later) consisted in teaching men by his example to express scientific thought in the German language. Among the opponents of Wolff are to be mentioned Lange, Crusius, Daries, and Euler. More or less nearly related to Wolff are Baumeister, Baumgarten, Meier, Reimarus,' and Ploucquet.2 Lambert, with much which lacks substance and logical form, gives much that has meaning and originality. His Neues Organon is divided into four parts, which Lambert callsDianoiologie, Alethiologie, Semiotik, and Phänomenologie. According to his explanation, they comprehend more completely what Aristotle and, after him, Bacon have called an organon. These sciences are instrumental,' or are instruments of the human understanding in the examination of truth. Dianoiologie is, according to Lambert, the doctrine of the laws of thought which the understanding must follow if it would advance from truth to truth. Alethiologie is the doctrine of truth, in so far as it is opposed to error, of the possibility of knowing truth. Semiotik is the doctrine of the expression of thought (especially of its expression in language). Phänomenologie is the doctrine of error, and of the means of avoiding it. Bilfinger (who wished also a logical theory for the subordinate cognitive faculties'), Feder, Eberhard," and Ernst Platner proceeded more or less on Leibnizian principles.

6

6

1 Vernunftlehre, 1756; 5th ed. 1790.

2 Methodus calculandi in Logicis, 1753; Methodus tam demonstrandi Omnes Syllogismorum Species, quam Vitiu Formae detegendi ope unius Regulae, 1763.

3 Leipzig, 1764.

4 Grundsätze der Logik und Metaphysik, 1769, and Institutiones Logicae et Metaphysicae, 1777.

5 Allgemeine Theorie des Denkens und des Empfindens, 1776.

6 Philo. Aphorismen, 1776, and Lehrbuch der Log. und Metaph.

1795.