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on the three other senses are quite different. I take in my hand a ball of glass; to the touch, it is the same as a ball of polished stone, and might recall the remembrance of such a ball if I had chanced to have been previously cognizant of one; but when I look at it, and hear the ring that it makes on being struck, the disparity is notable in both points, and would probably prevent my getting upon the old track of the marble specimen. The most impressive feature of the object. being its brilliant effect on the eye, this would have every chance to rule the identifying operation, and prevent me from recalling an object entirely destitute of this peculiarity. There might, however, be circumstances to carry my attention off from this effect, in which case the round smooth touch might start forth to the dignity of striking the recall.

In the popular classifications made among familiar objects, the identifying process is seen habitually at work. Looking out on the landscape, we observe an elevation of the ground, or an ascent from the ordinary level to a high point or peak; we note this appearance repeated under a great variety of shapes, and in different situations; we are not prevented by the disparity from recognizing the sameness; and every new individual, by similarity, re-instates the old. We thus bring together in the mind an array of objects widely scattered in nature; we give them a common name, mountain; we predicate of each new example the peculiarities that we have found attaching to the previous; we then know, without a trial, that if we were to ascend any one, we should experience a wide prospect, a diminishing temperature, and an altered vegetation.

In the same way, and with similar consequences, do we classify numerous other groups of natural objects;-cities, seas, lakes, rivers, forests, cultivated fields, quadrupeds, birds, fishes, &c. Natural History improves upon the popular classifications; it both searches the globe for materials, and subjects them to careful comparison. The progress of Natural History knowledge has been partly in the number of objects discovered, but partly also in the transition from superficial to

deep identities. In the time of Aristotle, animals were classified according to the element they inhabited; one class dwelling on the land, another in the sea, a third in the air: this point of identity being so prominent and forcible that it arrested every one's attention. Each of these classes could be subdivided by forming minor groups on still closer resemblances; thus, we should have, on the Earth, bipeds, quadrupeds, reptiles, &c., each of these groups being the assemblage of a number of individuals recalled to the view by special identities. So in the Air, the insect multitude would be readily marked off from the feathered tribes. It was not difficult to form classes such as these. But, more profound enquiry has developed features of identification carrying with them a greater amount of agreement, and on points of more value as knowledge, than in those ancient groupings. Birds are now identified, not by the circumstance of their flying in the air, but on the fact of their bringing forth their young in the egg, by their feathered structure, their warm-blooded circulation, &c. Instead of the old group of quadrupeds or animals walking on all fours, we have the class mammalia (which suckle their young), including both man and quadrupeds, and certain animals of the sea and the air.

24. The operation of Similarity in such classifyings and re-classifyings as the above, has a very high interest; it sets forth the workings of genius, and the history of science, and of the human mind. The reader has not as yet been prepared for fully carrying out this explanation. It is necessary first to dwell upon less complicated instances. I might follow the order adopted in developing the Law of Contiguity, and specify instances of the aggregation of impressions of the various senses the Organic sensibility with Taste, Smell, Touch, Hearing, or Sight; and it would be easy to lay hold of many cases of identity in diversity among such aggregates. Things affecting the palate alike may yet be very different to the touch and the sight, as in the different varieties of the same alimentary substances,-bread, butter, flesh, &c. Objects

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that are identical to the eye may yet differ to the taste and the smell, as water, alcohol, and white vinegar. We make a class founded on the common peculiarity, and give a designation implying that, and no more. If, however, the taste or smell is the point we are bent on studying, we do not pass from vinegar to water, but to other sour bodies, as the common acids.

25. Without pursuing farther the instances of aggregate impressions on a plurality of senses, let us next advert to the compounds of Sense and Association. Tools, implements, machinery, and all objects of practical utility, make a class that may stand first in exemplifying this aggregation. A knife, for example, is not simply an object of the senses; it is this and something more. Along with the sensation that it produces on the touch and the sight, there is an associated impression of its use, or of the cutting operation: and we are almost unable to regard it apart from this other circumstance. The appearance of a knife lying on the table is not the whole knife: the appearance of it in the hand while we feel its form and dimensions, coupling sight and touch, is not the whole knife; they are at best but signs or suggestive particulars that revive in the mind, by association, the full notion of the object. Here, therefore, we have a complication of sense and intellect, of impressions made by an actual object, with ideal or associated impressions, arising from previous junctures when we have seen it put to its use. In this association of sensible appearance with use, the last being only occasionally seen in the reality, and therefore for the most part an idea, or a potentiality, we have abundant room for the exercise of tracing likeness yoked with unlikeness. We may have similarity in form with diversity of use, and similarity of use with diversity of form. A rope suggests other ropes and cords, if we look to the appearance; but looking to the use, it may suggest an iron cable, a wooden prop, an iron girding, a leather band, or bevelled gear. In spite of diversity of appearance, the suggestion turns on what answers a common end. If we are

very much attracted by sensible appearances, there will be the more difficulty in recalling things that agree only in the use; if, on the other hand, we are profoundly sensitive to the one point of practical efficiency as a tool, the peculiarities not essential to this will be little noticed, and we shall be ever ready to revive past objects corresponding in use to some one present, although diverse in all other circumstances. We become oblivious to the difference between a horse, a steamengine, and a waterfall, when our minds are engrossed with the one circumstance of moving power. The diversity in these had no doubt for a long time the effect of keeping back their first identification; and to obtuse intellects, this identification might have been for ever impossible. A strong concentration of mind upon the single peculiarity of mechanical force, and a degree of indifference to the general aspect of the things themselves, must conspire with the intellectual energy of resuscitation by similars, in order to summon together in the view three structures so different. We can see, by an instance like this, how new adaptations of existing machinery might arise in the mind of a mechanical inventor. When it first occurred to a reflecting mind that moving water had a property identical with human or brute force, namely, the property of setting other masses in motion, overcoming inertia and resistance,-when the sight of the stream suggested through this point of likeness the power of the animal,—a new addition was made to the class of prime movers, and when circumstances permitted, this power could become a substitute for the others. It may seem to the modern understanding, familiar with water wheels and drifting rafts, that the similarity here was an extremely obvious one. But if we put ourselves back into an early state of mind, when running water affected the mind by its brilliancy, its roar, and irregular devastation, we may easily suppose that to identify this with animal muscular energy was by no means an obvious effect. Doubtless when a mind arose, insensible by natural constitution to the superficial aspects of things, and having withal a great stretch of identifying intellect, such a comparison

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would then be possible. We may pursue the same example one stage further, and come to the discovery of steam power, or the identification of expanding vapour with the previously known sources of mechanical force. To the common eye, for ages, vapour presented itself as clouds in the sky; or as a hissing noise at the spout of a kettle, with the formation of a foggy curling cloud at a few inches' distance. The forcing up of the lid of a kettle may also have been occasionally observed. But how long was it, ere any one was struck with the parallelism of this appearance with a blast. of wind, a rush of water, or an exertion of animal muscle? The discordance was too great to be broken through by such a faint and limited amount of likeness. In one mind, however, the identification did take place, and was followed out into its consequences. The likeness had occurred to other minds previously, but not with the same results. Such minds must have been in some way or other distinguished above the millions of mankind; and we are now endeavouring to give the explanation of their superiority. The intellectual character of Watt contained all the elements preparatory to a great stroke of similarity in such a case;a high susceptibility, both by nature and by education, to the mechanical properties of bodies; ample previous knowledge or familiarity; and indifference to the superficial and sensational effects of things. It is not only possible, however, but exceedingly probable, that many men possessed all these accomplishments; they are of a kind not transcending common abilities. They would in some degree attach to a mechanical education almost as a matter of course. That the discovery was not sooner made, supposes that something farther, and not of common occurrence, was necessary; and this additional endowment appears to be the identifying power of Similarity in general; the tendency to detect likeness in the midst of disparity and disguise. This supposition accounts for the fact; and is consistent with the known intellectual character of the inventor of the steamengine.