The philosophy of Hume imderwent three revisions, with, however, scarcely any essential change. It has been customary to enlarge upon the acumen and logical precision of Hume, but these qualifications resolve them- selves, on close scrutiny, into mere dialectical subtlety. If his artifices imposed upon others, he was often the victim of them himself, and he was crushed to the earth beneath the nuns of the systems which he overthrew, Hume's fundamental thesis is that all human knowledge (no pun is designed) consists of impressions and ideas. Impressions are the direct perceptions of sense : ideas are only the relics or signs of former impressions. Impres- sions are always particular, and incapable of variation : ideas are consequently the unalterable spectres of for- mer sensations. The theory of Locke is accepted and simphfied by discarding the oflice of reflection. The theory of Berkeley is accepted and expanded l)y apply- ing his argimient against matter to mind, and denj-ing all e^^dence of the existence of either. The result is a thoroughly Pyrrhonistic doubt. The application of these postulates, for postulates they are, generated the whole philosophy of Hume. There are only two objects of knowledge— the relations of ideas, and the relations of impressions or facts. The former relations are con- cerned Avith unchanging signs, and are therefore simple, and readily discerned by the discursion of thought; but the latter always involve the principle of cause and ef- fect, because due to some exciting influence. The rela- tion of cause and effect is nothing more than the habit- ual succession of events ; because all our complex con- ceptions are linked together only by customarj^ associa- tion, and it is impossible that particular objects should i produce a general idea. General ideas are, indeed, im-
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possibilities, for all abstrcactions are only vague images 1 of particulars. Ideas may represent either realities or phenomena, but no investigations can reach beyond the phenomenon to the reality. This reality is a pure de- lusion— a figment ; it is only the name arbitrarily given to a system of connected impressions and ideas. There is neither reality nor substance, neither matter nor mind; at least, there is nothing to authorize the asser- tion of their existence except as factitious phenomena. The connection of phenomena, or of the conceptions cor- responding with them, is accepted as truth in conse- quence of a primordial tendency of the mind, called be- lief. This beUef, however, imports nothing more than the tenacity of certain notions in consequence of the vivacity of the impressions by which they are produced. The credibility of facts is thus resolved into their appre- hensibility, and becomes merely a question of probabil- ities. This constitution of beUef, and this complexion of knowledge, result from the mode in which the mate- rials of thought are obtained. They are gathered by observation and experience, and are distinguished into two, and only two classes, according to their relative strength — i7nj)ressions and ideas; the former being the primary and more forcible perceptions ; the latter being the derivative and weaker, and being only copies of im- pressions. Fiu-ther than this it is impossible to carry speculation. The mind, the instrument of thought, lies beyond ; but its nature is discernible only in its opera- tions, and these constitute its whole nature so far as any attainable knowledge is concerned. Thus the human mind is the mould and measure of all knowledge, and yet that mind is itself only a problematical phenome- non. A good-humored scepticism is accordingly the sole result of philosophy.
From tills brief and imperfect sjmopsis of Hume's doc- trine— so well summed up by Slackintosh : " He aimed at proving, not that nothing was known, but that nothijig could be known" — it is easy to recognise the mode in which he reached its most startling applications. He might assert the moral sense, but the assertion was nu- gatory, for there could be no foundation for morals, nor anything more valid than expediencies growuig out of particular impressions and their observed sequences. He might admit the possibility, even the probability, of divine intelligence, but could not tell whether it was " ane or mair" since revelation could not be substituted for sensible perceptions. The scheme had no room for the admission of miracles, as they were unsupported by ordinary' experience, and human testimony was falla- cious. All this mischievous error is the appropriate fruit of the tree on which it hangs. Many refutations of these positions have been attempted, and a vigorous warfare has been waged on the principles supposed to form the foundation of this philosophy; but too little attention has been paid to the ambiguity of the terms employed, and to the vacillation with which they are used by the conjuror. A strict definition of " miracles" and " experience," and a rigid adherence to such defini- tion, will reduce the celebrated argument against mira- cles to a bald petitio primipii, or to a manifest absurdity. Hume endeavored to prove that " no testimony is suffi- cient to establish a miracle," and the reasoning employ- ed for this purpose is, that '• a miracle being a violation of the laws of nature, which a firm and unalterable ex- perience has established, the proof against a miracle, from the very natiu-e of the fact, is as entire as any ar- gument from experience can be; whereas our experi- ence of human veracity, which (according to him) is the sole foundation of the evidence of testimony, is far from being uniform, and can, therefore, never preponderate against that experience which admits of no exception." This boasted and plausible argument has, with equal candor and acuteness, been examined by Dr. Campbell, in his Dissertation on Miracles, who justly observes that, so far is experience from being the sole foundation of the evidence of testimony, that, on the contran,', testi- mony is the sole foundation of by far the greater part