and of time, arising from our always measuring the one of these quantities by the other. We measure time by motion, and motion by extenfion. In an hour, the hand of the clock moves over a certain space; in two hours, over double the fpace; and fo on. Hence the ideas of space and of time become very intimately united, and we apply to the latter the words long and short, before and after, in the same manner as to the former. The apprehended analogy between the relation which the different notes in the fcale of mufic bear to each other, and the relation of fuperiority and inferiority, in respect of pofition, among material objects, arifes alfo from an accidental affociation of ideas. What this affociation is founded upon, I fhall not take upon me to determine; but that it is the effect of accident, appears clearly from this, that it has not only been confined to particular ages and nations; but is the very reverfe of an affociation which was once equally prevalent. It is obferved by Dr. Gregory, in the preface to his edition of Euclid's works, that the more ancient of the Greek writers looked upon grave founds as high, and acute ones as low; and that the prefent mode of expreffion on that fubject, was an innovation introduced at a later period.* In the inftances which have now been mentioned, our habit of combining the notions of two things, becomes fo ftrong, that we find it impoffible to think of the one, without thinking at the fame time of the other. Various other examples of the fame fpecies of combination, although, perhaps, not altogether so striking in degree, might eafily be collected from the fubjects about which our metaphyfical fpeculations are employed. The fenfations, for inftance, * See Note [Q.] which are excited in the mind by external objects, and the perceptions of material qualities which follow thefe fenfations, are to be diftinguished from each other only by long habits of patient reflection. A clear conception of this diftinction may be regarded as the key to all Dr. Reid's reafonings concerning the process of nature in perception; and, till it has once been rendered familiar to the reader, a great part of his writings must appear unfatisfactory and obfcure.-In truth, our progrefs in the philofophy of the human mind depends much more on that severe and difcriminating judgment, which enables us to separate ideas which nature or habit have intimately combined, than on acuteness of reasoning or fertility of invention. And hence it is, that metaphyf ical ftudies are the best of all preparations for those philofophical purfuits which relate to the conduct of life. In none of these do we meet with cafual combinations fo intimate and indiffoluble as those which occur in metaphyfics; and he who has been accuftomed to fuch difcriminations as this fcience requires, will not easily be impofed on by that confufion of ideas, which warp the judgments of the multitude in moral, religious, and political inquiries. From the facts which have now been stated, it is eafy to conceive the manner in which the affociation of ideas has a tendency to mislead the judgment, in the first of the three cafes already enumerated. When two fubjects of thought are fo intimately connected together in the mind, that we find it scarcely poffible to consider them apart; it must require no common efforts of attention, to conduct any process of reafoning which relates to either. I formerly took notice of the errors to which we are expofed in confequence of the ambiguity of words; and of the neceffity of frequently checking and correcting our general reafonings by means of particular examples; but in the cafes to which I allude at present, there is (if I may use the expreffion) an ambiguity of things; fo that even when the mind is occupied about particulars, it finds it difficult to feparate the proper object of its attention from others with which it has been long accustomed to blend them. The cafes, indeed, in which fuch obftinate and invincible affociations are formed among different fubjects of thought, are not very numerous, and occur chiefly in our metaphysical researches; but in every mind, cafual combinations, of an inferior degree of ftrength, have an habitual effect in disturbing the intellectual powers, and are not to be conquered without perfevering exertions, of which few men are capable. The obvious effects which this tendency to combination produces on the judgment, in confounding together those ideas which it is the province of the metaphyfician to diftinguish, fufficiently illuftrate the mode of its operation in those numerous inftances, in which its influence, though not fo complete and ftriking, is equally real, and far more dangerous. II. The affociation of ideas is a fource of fpeculative error, by misleading us in those anticipations of the future from the paft, which are the foundation of our conduct in life. The great object of philofophy, as I have already remarked more than once, is to afcertain the laws which regulate the fucceffion of events, both in the phyfical and moral worlds; in order that, when called upon to act in any particular combination of circumstances, we may be enabled to anticipate the probable courfe of nature from our paft experience and to regulate our conduct accordingly. As a knowledge of the established connexions among events, is the foundation of fagacity and of kill, both in the practical arts, and in the conduct of life, nature has not only given to all men a ftrong difpofition to remark, with attention and curiofity, thofe phenomena which have been observed to hap pen nearly at the fame time; but has beautifully adapted to the uniformity of her own operations, the laws of affociation in the human mind. By rendering contiguity in time one of the ftrongest of our affociating principles, fhe has conjoined together in our thoughts, the fame events which we have found conjoined in our experience, and has thus accommodated (without any effort on our part) the order of our ideas to that scene in which we are deftined to act. The degree of experience which is neceffary for the preservation of our animal existence, is acquired by all men without any particular efforts of study. The laws of nature, which it is most material for us to know, are expofed to the immediate observation of our fenfes; and establish, by means of the principle of affociation, a correfponding order in our thoughts, long before the dawn of reafon and re-. flection; or at least long before that period of childhood, to which our recollection afterwards extends. This tendency of the mind to affociate together events which have been prefented to it nearly at the fame time; although, on the whole, it is attended with infinite advantages, yet, like many other principles of our nature, may occafionally be a fource of inconvenience, unless we avail ourselves of our reafon and of experience in keeping it under proper regulation. Among the various phenomena which are continually paffing before us, there is a great proportion, whofe vicinity in time does not indicate a conftancy of conjunction; and unless we be careful to make the diftinction between these two claffes of connections, the order of our ideas will be apt to correfpond with the one as well as with the other; and our unenlightened experience of the paft, will fill the mind, in numberiefs inftances, with vain expectations, or with groundless alarms, concerning the future. This difpofition to confound together acci dental and permanent connections, is one great fource of popular fup rftitions. Hence the regard which is paid to unlucky days; to unlucky colours; and to the influence of the planets; apprehenfions which render human life, to many, a continued series of abfurd terrors. Lucretius compares them to those which children feel, from an idea of the existence of spirits in the dark: "Ac veluti pueri trepidant, atque omnia cœcis Such spectres can be difpelled by the light of phi lofophy only; which, by accuftoming us to trace established connections, teaches us to defpife those which are cafual; and, by giving a proper direction to that bias of the mind which is the foundation of fuperftition, prevents it from leading us aftray. In the inftances which we have now been confidering, events come to be combined together in the mind, merely from the accidental circumftance of their contiguity in time, at the moment when we perceived them. Such combinations are confined, in a great measure, to uncultivated and unenlightened minds; or to those individuals who, from nature or education, have a more than ordinary facility of affociation. But there are other accidental combinations, which are apt to lay hold of the most vigorous understandings; and from which, as they are the natural and neceffary refult of a limited experience, no fuperiority of intellect is fufficient to preferve a philofopher, in the infancy of phyfical fcience. As the connections among phyfical events are diacovered to us by experience alone, it is evident, that when we see a phenomenon preceded by a number of different circumstances, it is impoffible for us to determine, by any reasoning a priori, which of these |