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we conceive. Something perfectly analogous to this may be remarked in the perceptions we obtain by the sense of fight. When I look into a shew-box, where the deception is imperfect, I see only a set of paltry dawbings of a few inches diameter ; but, if the representation be executed with so much skill, as to convey to me the idea of a distant prospect, every object before me swells in its dimensions, in proportion to the extent of space which I conceive it to occupy; and what seemed before to be shut up within the limits of a small wooden frame, is magnified, in my apprehension, to an immense landscape of woods, rivers, and mountains.

The phenomena which we have hitherto explained, take place when sleep seems to be complete; that is, when the mind loses its influence over all those powers whose exercise depends on its will. There are, however, many cases in which sleep seems to be partial ; that is, when the mind loses its influence over fome powers, and retains it over others. In the case of the fomnambuli, it retains its power over the limbs, but it possesses no influence over its own thoughts, and scarcely any over the body; excepting those particular members of it which are employed in walking. In madness, the power of the will over the body remains undiminished, while its influence in regulating the train of thought is in a great measure suspended ; either in consequence of a particular idea, which engrosses the attention, to the exclusion of every thing else, and which we find it impossible to banish by our efforts; or in consequence of our thoughts succeeding each other with such rapidity, that we are unable to stop the train. In both

of

of these kinds of madness, it is worthy of remark, that the conceptions or imaginations of the mind becoming independent of our will, they are apt to be mistaken for actual perceptions, and to affect us the same in

manner.

By means of this supposition of a partial sleep, any apparent exceptions which the history of dreams may afford to the general principles already stated, admit of an easy explanation.

Upon reviewing the foregoing observations, it does not occur to me, that I have in any instance transgressed those rules of philosophising, which, since the time of Newton, are commonly appealed to, as the tests of found investigation. For, in the first place, I have not supposed any causes which are not known to exist; and secondly, I have shewn, that the phenomena under our consideration are necessary consequences of the causes to which I have referred them. I have not supposed, that the mind acquires in sleep, any new faculty of which we are not conscious while awake; but only (what we know to be a fact) that it retains some of its powers, while the exercise of others is fufpended : and I have deduced synthetically, the known phenomena of dreaming, from the operation of a particular class of our faculties, uncorrected by the operation of another. I flatter myself, therefore, that this inquiry will not only throw some light on the state of the mind in sleep; but that it will have a tendency to illustrate the mutual adaptation and subserviency which exists among the different parts of our constitution, when we are in complete possession of all the faculties and principles which belong to our nature *.

* See Note [O].

CHAPTER FIFTH.

PART SECOND.

Of the influence of Association on the Intellectual and

on the Active Powers.

SECTION I.

of the Influence of casual Asociations on our speculative

Conclusions.

Association of ideas has a tendency to warp THE

our speculative opinions chiefly in the three fol. lowing ways:

First, by blending together in our apprehensions, things which are really distinct in their nature ; so as to introduce perplexity and error into every process of reasoning in which they are involved.

Secondly, by misleading us in those anticipations of the future from the past, which our constitution disposes us to form, and which are the great foundation of our conduct in life.

Thirdly, by connecting in the mind erroneous opinions, with truths which irresistibly command our assent, and which we feel to be of importance to human happiness.

A short illustration of these remarks, will throw light on the origin of various prejudices; and may, perhaps,

suggest

fuggest some practical hints with respect to the conduct of the understanding.

I.I formerly had occasion to mention several instances of very intimate associations formed between two ideas which have no neceffary 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 * ; and yet, in confequence 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

* See Note [P].

of

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 foort, 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 fuperiority and inferiority, in respect of position, among material objects, arises also from an accidental association of ideas.

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 *.

In the instances which have now been mentioned, our habits of combining the notions of two things, becomes 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

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