very intimate associations formed between two ideas which have no necessary connexion with each other. One of the most remarkable is, that which exists in every person's mind between the notions of colour and of extension. The former of these words expresses (at least in the sense in which we commonly employ it) a sensation in the mind, the latter denotes a quality of an external object; so that there is, in fact, no more connexion between the two notions than between those of pain and of solidity;1 and yet, in consequence of our always perceiving extension, at the same time at which the sensation of colour is excited in the mind, we find it impossible to think of that sensation without conceiving extension along with it.

Another intimate association is formed in every mind between the ideas of space and of time. When we think of an interval of duration, we always conceive it as something analogous to a line, and we apply the same language to both subjects. We speak of a long and short time, as well as of a long and short distance, and we are not conscious of any metaphor in doing so. Nay, so very perfect does the analogy appear to us, that Boscovich mentions it as a curious circumstance, that extension should have three dimensions, and duration only one.

This apprehended analogy seems to be founded wholly on an association between the ideas of space and of time, arising from our always measuring the one of these quantities by the other. We measure time by motion, and motion by extension. In an hour, the hand of the clock moves over a certain space; in two hours, over double the space, and so 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 scale of music bear to each other, and the relation of superiority and inferiority in respect of position among material objects, arises also from an accidental association of ideas.

1 See Note P.

What this association is founded upon, I shall 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 reverse of an association. which was once equally prevalent. It is observed by Dr. Gregory, in the preface to his edition of Euclid's works, that the more ancient of the Greek writers looked upon grave sounds as high, and acute ones as low, and that the present mode of expression on that subject was an innovation introduced at a later period.1

In the instances which have now been mentioned, our habits of combining the notions of two things become so strong, that we find it impossible to think of the one, without thinking at the same time of the other. Various other examples of the same species of combination, although perhaps not altogether so striking in degree, might easily be collected from the subjects about which our metaphysical speculations are employed. The sensations, for instance, which are excited in the mind by external objects, and the perceptions of material qualities which follow these sensations, are to be distinguished from each other only by long habits of patient reflection. A clear conception of this distinction may be regarded as the key to all Dr. Reid's reasonings 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 unsatisfactory and obscure. In truth, our progress in the philosophy of the human mind depends much more on that severe and discriminating judgment, which enables us to separate ideas which nature or habit have immediately combined, than on acuteness of reasoning or fertility of invention. And hence it is, that metaphysical studies are the best of all preparations for those philosophical pursuits which relate to the conduct of life. In none of these do we meet with casual combinations so intimate and indissoluble as those which occur in metaphysics, and he who has been accustomed to such discriminations as this science requires, will not easily be imposed on by that confusion of

1 See Note Q.

ideas which warps the judgments of the multitude in moral, religious, and political inquiries.

From the facts which have now been stated, it is easy to conceive the manner in which the association of ideas has a tendency to mislead the judgment in the first of the three cases already enumerated. When two subjects of thought are so intimately connected together in the mind, that we find it scarcely possible to consider them apart, it must require no common efforts of attention to conduct any process of reasoning which relates to either. I formerly took notice of the errors to which we are exposed in consequence of the ambiguity of words; and of the necessity of frequently checking and correcting our general reasonings by means of particular examples; but in the cases to which I allude at present, there is (if I may use the expression) an ambiguity of things; so that even when the mind is occupied about particulars, it finds it difficult to separate the proper objects of its attention from others with which it has been long accustomed to blend them. The cases, indeed, in which such obstinate and invincible associations are formed among different subjects of thought, are not very numerous, and occur chiefly in our metaphysical researches; but in every mind casual combinations of an inferior degree of strength have an habitual effect in disturbing the intellectual powers, and are not to be conquered without persevering 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 metaphysician to distinguish, sufficiently illustrate the mode of its operation in those numerous instances in which its influence, though not so complete and striking, is equally real, and far more dangerous.

II. The association of ideas is a source of speculative error, by misleading us in those anticipations of the future from the past, which are the foundation of our conduct in life.

The great object of philosophy, as I have already remarked more than once, is to ascertain the laws which regulate the succession of events, both in the physical 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 course of nature from our past experience, and to regulate our conduct accordingly.

As a knowledge of the established connexions among events is the foundation of sagacity and of skill, both in the practical arts and in the conduct of life, nature has not only given to all men a strong disposition to remark, with attention and curiosity, those phenomena which have been observed to happen nearly at the same time, but has beautifully adapted to the uniformity of her own operations, the laws of association in the human mind. By rendering contiguity in time one of the strongest of our associating principles, she has conjoined together in our thoughts the same 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 destined to act.

The degree of experience which is necessary 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 exposed to the immediate observation of our senses, and establish, by means of the principle of association, a corresponding order in our thoughts, long before the dawn of reason and reflection, or at least long before that period of childhood to which our recollection afterwards extends.

This tendency of the mind to associate together events which have been presented to it nearly at the same time, although on the whole it is attended with infinite advantages, yet, like many other principles of our nature, may occasionally be a source of inconvenience, unless we avail ourselves of our reason and of our experience in keeping it under proper regulation. Among the various phenomena which are continually passing before us, there is a great proportion whose vicinity in time does not indicate a constancy of conjunction; and unless we be careful to make the distinction between these two classes of connexions, the order of our ideas will be apt to correspond

with the one as well as with the other, and our unenlightened experience of the past will fill the mind, in numberless instances, with vain expectations, or with groundless alarms concerning the future. This disposition to confound together accidental and permanent connexions, is one great source of popular superstitions. Hence the regard which is paid to unlucky days, to unlucky colours, and to the influence of the planets; apprehensions which render human life to many a continued series of absurd 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 cæcis

In tenebris metuunt, sic nos in luce timemus,
Interdum nihilo quæ sunt metuenda magis."

Such spectres can be dispelled by the light of philosophy only, which, by accustoming us to trace established connexions, teaches us to despise those which are casual; and, by giving a proper direction to that bias of the mind which is the foundation of superstition, prevents it from leading us astray.

In the instances which we have now been considering, events come to be combined together in the mind merely from the accidental circumstance 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 association. 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 necessary result of a limited experience, no superiority of intellect is sufficient to preserve a philosopher in the infancy of physical science.

As the connexions among physical events are discovered to us by experience alone, it is evident that, when we see a phenomenon preceded by a number of different circumstances, it is impossible for us to determine, by any reasoning a priori, which of these circumstances are to be regarded as the constant, and