general principles; and by means of these it can summon up, as occasion may require, an infinite number of particulars associated with them, each of which, considered as a solitary truth, would have been as burdensome to the memory as the general principle with which it is connected.1

I would not wish it to be understood from these observations, that philosophy consists in classification alone, and that its only use is to assist the memory. I have often, indeed, heard this asserted in general terms; but it appears to me to be obvious, that although this be one of its most important uses, yet something more is necessary to complete the definition of it. Were the case otherwise, it would follow that all classifications are equally philosophical, provided they are equally comprehensive. The very great importance of this subject will, I hope, be a sufficient apology for me in taking this opportunity to correct some mistaken opinions which have been formed concerning it.

SECT. V.-CONTINUATION OF THE SAME SUBJECT.

AID WHICH THE MEMORY DERIVES FROM PHILOSOPHICAL ARRANGEMENT.

It was before observed that the great use of the faculty of memory, is to enable us to treasure up for the future regula

[It is very justly and ingeniously remarked by Dr. Priestley, that the more we know of any branch of science, the less is the compass into which we are able to bring its principles, provided the facts from which they are inferred be numerous.' The reason is, that "in an advanced state of knowledge, we are able to reduce more of the particular into general observations; whereas, in the infancy of a science, every observation is an independent fact, and in delivering the principles of it, they must all be distinctly mentioned, so that, though a selection may be made, a proper abridgment is impossible."

In illustration of this, the same author

observes farther, that "Notwithstanding the vast additions that have been made to the science of optics within the last hundred years, a judicious summary of the whole will be much shorter now than it would have been a century ago; and yet it is probable, much larger than there will be any necessity of making it a century hence; as it may be presumed, that by that time a connexion will be traced between many facts which now appear to be unconnected and independent of one another, and therefore require to be recited separately."— History of Discoveries relating to Vision, &c., p. 768.]

tion of our conduct, the results of our past experience, and of our past reflections. But in every case in which we judge of the future from the past, we must proceed on the belief that there is in the course of events, a certain degree at least of uniformity. And accordingly, this belief is not only justified by experience, but (as Dr. Reid has shewn in a very satisfactory manner) it forms a part of the original constitution of the human mind. In the general laws of the material world, this uniformity is found to be complete; insomuch that in the same combinations of circumstances, we expect, with the most perfect assurance, that the same results will take place. In the moral world, the course of events does not appear to be equally regular, but still it is regular, to so great a degree as to afford us many rules of importance in the conduct of life.

A knowledge of nature, in so far as it is absolutely necessary for the preservation of our animal existence, is obtruded on us, without any reflection on our part, from our earliest infancy. It is thus that children learn of themselves to accommodate their conduct to the established laws of the material world. In doing so they are guided merely by memory, and the instinctive principle of anticipation, which has just been mentioned.

In forming conclusions concerning future events, the philosopher as well as the infant can only build with safety on past experience; and he, too, as well as the infant, proceeds on an instinctive belief, for which he is unable to account, of the uniformity of the laws of nature. There are, however, two important respects which distinguish the knowledge he possesses from that of ordinary men. In the first place, it is far more extensive, in consequence of the assistance which science gives to his natural powers of invention and discovery. Secondly, it is not only more easily retained in the memory, and more conveniently applied to use, in consequence of the manner in which his ideas are arranged; but it enables him to ascertain by a process of reasoning, all those truths which may be synthetically deduced from his general principles. The illustration of these particulars will lead to some useful remarks, and will at the same time shew, that in discussing the subject

of this Section, I have not lost sight of the inquiry which occasioned it.

I.-1. It was already remarked, that the natural powers of Memory, together with that instinctive anticipation of the future from the past, which forms one of the original principles of the mind, are sufficient to enable infants, after a very short experience, to preserve their animal existence. The laws of nature, which it is not so important for us to know, and which are the objects of philosophical curiosity, are not so obviously exposed to our view, but are, in general, brought to light by means of experiments which are made for the purpose of discovery; or, in other words, by artificial combinations of circumstances, which we have no opportunity of seeing conjoined in the course of our ordinary experience. In this manner, it is evident that many connexions may be ascertained, which would never have occurred spontaneously to our observation.

2. There are, too, some instances, particularly in the case of the astronomical phenomena, in which events that appear to common observers to be altogether anomalous, are found, upon a more accurate and continued examination of them, to be subjected to a regular law. Such are those phenomena in the heavens, which we are able to predict by means of cycles. In the cases formerly described, our knowledge of nature is extended by placing her in new situations. In these cases, it is extended by continuing our observations beyond the limits of ordinary curiosity.

3. In the case of human affairs, as long as we confine our attention to particulars, we do not observe the same uniformity as in the phenomena of the material world. When, however, we extend our views to events which depend on a combination of different circumstances, such a degree of uniformity appears, as enables us to establish general rules, from which probable conjectures may often be formed with respect to futurity. It is thus that we can pronounce, with much greater confidence, concerning the proportion of deaths which shall happen in a certain period among a given number of men, than we can predict the death of any individual; and that it is more reasonable

to employ our sagacity in speculating concerning the probable determinations of a numerous society, than concerning events which depend on the will of a single person.

In what manner this uniformity in events depending on contingent circumstances is produced, I shall not inquire at present. The advantages which we derive from it are obvious, as it enables us to collect, from our past experience, many general rules, both with respect to the history of political societies, and the characters and conduct of men in private life.

4. In the last place; the knowledge of the philosopher is more extensive than that of other men, in consequence of the attention which he gives, not merely to objects and events, but to the relations which different objects and different events bear to each other.

The observations and the experience of the vulgar are almost wholly limited to things perceived by the senses. A similarity between different objects, or between different events, rouses their curiosity, and leads them to classification and to general rules. But a similarity between different relations is seldom to be traced without previous habits of philosophical inquiry. Many such similarities or connexions, however, are to be found in nature; and when once they are ascertained, they frequently lead to important discoveries, not only with respect to other relations, but with respect to the objects or to the events which are related. These remarks it will be necessary to illustrate more particularly.

The great object of Geometry is to ascertain the relations. which exist between different quantities, and the connexions which exist between different relations. When we demonstrate that the angle at the centre of a circle is double of the angle at the circumference on the same base, we ascertain a relation between two quantities. When we demonstrate that triangles of the same altitude are to each other as their bases, we ascertain a connexion between two relations. It is obvious how much the mathematical sciences must contribute to enlarge our knowledge of the universe in consequence of such discoveries. In that simplest of all processes of practical geometry, which

teaches us to measure the height of an accessible tower, by comparing the length of its shadow with that of a staff fixed vertically in the ground, we proceed on the principle, that the relation between the shadow of the staff and the height of the staff is the same with the relation between the shadow of the tower and the height of the tower. But the former relation we can ascertain by actual measurement; and, of consequence, we not only obtain the other relation, but as we can measure one of the related quantities, we obtain also the other quantity. In every case in which mathematics assists us in measuring the magnitudes or the distances of objects, it proceeds on the same principle; that is, it begins with ascertaining connexions among different relations, and thus enables us to carry our inquiries from facts which are exposed to the examination of our senses, to the most remote parts of the universe.

I observed, also, that there are various relations existing among physical events, and various connexions existing among these relations. It is owing to this circumstance that mathematics is so useful an instrument in the hands of the physical inquirer. In that beautiful theorem of Huygens, which demonstrates that the time of a complete oscillation of a pendulum in the cycloid, is to the time in which a body would fall through the axis of the cycloid, as the circumference of a circle is to its diameter, we are made acquainted with a very curious and unexpected connexion between two relations; and the knowledge of this connexion facilitates the determination of a most important fact with respect to the descent of heavy bodies near the earth's surface, which could not be ascertained conveniently by a direct experiment.

In examining, with attention, the relations among different physical events, and the connexions among different relations, we sometimes are led by mere induction to the discovery of a general law, while, to ordinary observers, nothing appears but irregularity. From the writings of the earlier opticians we learn, that, in examining the first principles of dioptrics, they were led by the analogy of the law of reflection, to search for the relation between the angles of incidence and refraction, (in