PHILOSOPHY, MENTAL. 



529 



and Stewart). Rejecting innate ideas, Locke teaches 

 that sensation and reflection are the only sources of 

 knowledge, external objects furnishing the mind 

 with the ideas of sensible qualities, and the mind 

 furnishing the understanding with ideas of its own 

 operations. Sensation convinces us of the existence 

 of solid extended substance, and reflection of the 

 existence of thinking ones, of the cause and nature 

 of which two kinds of being we can know nothing. 

 Perception is a communication between the mind and 

 external objects carried on by means of images pre- 

 sent to the mind ; these he calls ideas, which he de- 

 fines to be the immediate objects about which the 

 mind is employed in thinking. Having treated at 

 length of the origin, nature and qualities of ideas, he 

 proceeds to consider the instrument by which men 

 communicate their ideas to each other ; and his re- 

 marks on this subject (Book iii., of Language) form 

 the most valuable dogmatic part of his work. Know- 

 ledge is the perception of the agreement or disagree- 

 ment of ideas, which consists in identity or diversity, 

 relation, co-existence, and real existence. . Of the 

 existence of ourselves and of God we have intuitive 

 knowledge, which is the immediate perception of the 

 agreement or disagreement of ideas : demonstrative 

 knowledge is the discovery of it by the intermedi- 

 ation of other ideas : and these two sorts of knowledge 

 yield complete certainty. Sensitive knowledge leads 

 to the belief of the existence of other beings, and 

 carries with it a reasonable confidence. Judgment 

 is a supposition or opinion of the agreement or dis- 

 agreement of ideas, and supplies the want of know- 

 ledge. Its conclusions are only probable. The 

 ethical consequences which had been deduced from 

 the sensualist school of Hobbes, and from a partial 

 view of the doctrines of Locke, led Berkeley, who 

 was not less remarkable for the virtues of his char- 

 acter than for the acuteness of his philosophy, to the 

 adoption of idealism (Theory of Vision, 1709 ; Prin- 

 ciples of Human Knowledge, 1710; Alciphron, or 

 the Minute Philosopher, 1732). His Theory of Vi- 

 sion, which is the most valuable part of his labours, 

 and which is an important addition to the knowledge 

 of mind, was the first exposition of the difference be- 

 tween the original and acquired perceptions of the 

 eye, and now forms an essential part of the science 

 of optics. His scheme of idealism was founded on 

 the Lockian doctrine of ideas. Proceeding from the 

 principle, that we are percipient of nothing but our 

 own perceptions and ideas, and that all the objects 

 of human knowledge are ideas of sensation or reflec- 

 tion existing in the mind itself, he comes to the con- 

 clusion that the existence of bodies out of a mind 

 perceiving them, is not only impossible, and a con- 

 tradiction in terms ; but, were it possible, and even 

 real, it were impossible we should ever know it. By 

 thus " expelling matter out of nature," he thought 

 we should get rid of the chief cause of all error in 

 philosophy, and all infidelity in religion. Granting 

 the premises of Berkeley, which were the commonly 

 received philosophical views, at least in England, 

 his conclusions could not be refuted ; but it was re- 

 served for Hume to trace out, by a vigorous and un- 

 shrinking logic, the legitimate consequences of the 

 Cartesian and Lockian philosophy to their ultimate 

 results, and thus, though unintentionally, by a sort 

 of reductio ad absurdum, to produce the great meta- 

 physical revolution, of which Reid and Kant were 

 the first movers. This lie did with such power of 

 logic, acuteness and cogency of reasoning, boldness, 

 precision, clearness, and elegance, that scepticism 

 never appeared more formidable or more seducing 

 than in his writings (Treatise of Human Nature, 

 1738, cast anew in the Inquiry concerning Human 

 Understanding, 1748). After showing that all at- 



tempted demonstrations of the necessity of a cause to 

 every new existence are fallacious and sophistical, 

 Hume endeavours to prove that the proposition, 

 whatever has a beginning has a cause, is not intui- 

 tively certain, but is derived only from custom and 

 belief, and is rather an act of the sensitive than of 

 the cogitative part of our nature. In this argument, 

 he proceeds on the ground that all certainty arises 

 from a comparison of ideas, and the discovery of 

 their unalterable relations, which are resemblance, 

 proportions in quantity and number, degrees of qual- 

 ity and contrariety, and none of which is implied in 

 the proposition above stated. All the objects of 

 knowledge are impressions and ideas; the former 

 are our more lively perceptions, when we hear or 

 see, love or hate, or desire or will ; the latter are 

 the less lively perceptions of which we are conscious 

 when we reflect on the former, and are copies of im- 

 pressions. The existence of these perceptions as ob- 

 jects of consciousness cannot be denied ; but to admit 

 the existence of a percipient being, the I, is to assume 

 that of mind, which is no more an object of know- 

 ledge than matter. There can, therefore, be no ob- 

 jective knowledge ; and we are reduced to conscious- 

 ness, the phenomena of which it takes cognizance, 

 and their subjective relations. Hume's system of 

 scepticism is not scepticism antecedent to study and 

 philosophy, but consequent to science and inquiry, 

 holding the absolute fallaciousness of the mental 

 faculties, bringing the senses themselves into dispute, 

 and thus sapping the foundations of all knowledge, 

 and rejecting the existence of God, a providence, 

 and a future state. At about the same time, Hartley 

 (q. v.) attempted to account for all the phenomena 

 of the mind, by the single principle of the association 

 of ideas, and for this principle by vibrations and vib- 

 ratiuncles in the medullary substance of the brain. 

 In connexion with this plan of materialism, he de- 

 fended the doctrine of necessity, representing God as 

 the only cause of all natural effects and all human 

 actions. To the Hartleian school belong Priestley, 

 Darwin, and Home Tooke. The sceptical conclu- 

 sions which Hume had irresistibly shown to be the 

 result of the ideal system of philosophy, which had 

 been received since the time of Descartes and Locke, 

 led Reid (Inquiry into the Human Mind, 1764 ; On 

 the Intellectual Powers, 1785)-to the examination and 

 refutation of that system itself. The Scotch school 

 of philosophy, modest and perhaps timid in its pre- 

 tensions, has the merit of having first strongly and 

 largely inculcated the absolute necessity of admitting 

 certain principles as the foundation of all reasoning, 

 and as being the indispensable conditions of thought 

 itself. The Kantian philosophy is only a modifica- 

 tion of it. According to the Scotch philosophers, 

 certain simple ideas are implied and involved in cer- 

 tain intuitive judgments of the mind ; thus identity, 

 cause, time, number, truth, certainty, probability, 

 are ideas peculiar to a rational mind, and necessarily 

 arise in the human understanding, when employed in 

 the exercise of its different faculties. Reid, there- 

 fore, while he rejected the Cartesian theory of ideas 

 or images in the mind being the only objects of 

 thought, directed his inquiries to an analysis of the 

 various powers and principles of our constitution, in 

 order to discover the fundamental laws of belief, 

 which form the groundwork of human knowledge. 

 Though professing to build only on experience, he 

 did not limit experience to the relations of sense and 

 its objects. Without claiming for man more than a 

 relative knowledge of existence, and restricting the 

 science of mind to an observation of the fact of con 

 sciousness, he analyzed that fact into a greater num- 

 ber of more important elements than had been recog- 

 nised in the sensualist school. He showed that 



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