LECTURE V. ON THE NATURE OF PHYSICAL INQUIRY IN GENERAL. - THE preceding Lectures, gentlemen, have, I trust, sufficiently convinced of the importance of the science on which we are to enter, if, indeed, many of the advantages which we have considered were not of themselves so obvious, as readily to have occurred to your own reflection, or at least to require less illustration, than,-in my desire to interest not your attention merely, but your zealous ardour, in a science which appears to me so truly to deserve it, I have thought necessary to give them. We have seen, how interesting the mind is, as an object of study, from its own intrinsic excellence, even though it were to be considered in no other light than as a mere part of the universal system of things, necessary, therefore, to be comprehended with every other existing substance, in a system of general physics. We have seen likewise, in how many important respects, the study of the science of mind is favourable to the growth of virtuous sentiment, and to the refinement and happiness of society; and, above all, how essential an acquaintance with it is, to the proper conduct of our inquiries,—not merely in those sciences, the objects of which are kindred or analogous, but in every other science, the various objects of which, however independent, and even remote from it they may seem, must always be considered, not as they exist in themselves, but as they exist in relation to it; since they can be known to us only through the medium of the mental affections, or feelings, excited by them, which have laws peculiar to themselves, and analyzed and arranged only by our mental faculties, which have their own peculiar limits of extent and power. The first great division of our course of inquiry is purely physiological. It has for its object the mind, considered as susceptible of various states or affections, and constituting, as it is thus variously affected, the whole phenomena of thought and feeling, which, though expressed by a variety of terms, of functions, or faculties, are still but the one mind itself existing in different states. On retracing these states, which form the whole progress of our sentient, intellectual, and moral life, we have to inquire into the properties of the substance, mind, according to the same laws of investigation, by which we inquire into the properties of external substances,-not by assuming principles, from which the phenomena may be supposed to flow, but by observing and generalizing, till we arrive at those few simple principles or laws, which however pompous the term laws may seem, as if it denoted something different from the phenomena themselves, and paramount to them, are, in truth, nothing more than the expression of the most general circumstances, in which the phenomena themselves have been felt by us to agree. As we say of gold, that it is that which is of a certain specific weight, yellow, ductile, fusible at a certain temperature, and capable of certain combinations, because all these properties have been observed by ourselves or others, so we say of the mind, that it is that which perceives, remembers, compares, and is susceptible of various emotions or other feelings; because of all these we have been conscious, or have observed them indirectly in others. We are not entitled to state with confidence any quality, as a pro perty of gold, which we do not remember to have observed ourselves, or to have received on the faith of the observation of others, whose authority we have reason to consider as indubitable; and as little are we entitled to assert any quality, or general susceptibility, as belonging to the human mind, of which we have not been conscious ourselves in the feelings resulting from it, or for which we have not the authority of the indubitable consciousness of others. The exact coincidence, in this respect, of the physics of mind and of matter, it is important that you should have constantly before you, that you may not be led to regard the comparative indistinctness and vagueness of the mental phenomena as a warrant for greater boldness of assertion, and looseness of reasoning with respect to them. There is, on the contrary, in such a case, still greater reason to adhere rigidly to the strict rules of philosophizing; because the less definite the phenomena are, the greater danger is there of being misled in discriminating and classing them. The laws of inquiry, those general principles of the logic of physics, which regulate our search of truth in all things, external and internal, do not vary with the name of a science, or its objects or instruments. They are not laws of one science, but of every science, whether the objects of it be mental or material, clear or obscure, definite or indefinite; and they are thus universal, because, in truth, though applicable to many sciences, they are only laws of the one inquiring mind, founded on the weakness of its powers of discernment, in relation to the complicated phenomena on which those powers are exercised. The sort of reasoning which would be false in chemistry, would be false in astronomy, would be false in the physiology of our corporeal or intellectual and moral nature, and in all, for the same reason; because the mind is the inquirer in all alike, and is limited, by the very constitution of its faculties, to a certain order of inquiry, which it must, in this case of supposed erroneous reasoning, have transgressed. On these general laws of inquiry, as relating alike to the investigation of the properties of matter and of mind, it is my intention to dwell, for some time, with full discussion; for, though the subject may be less pleasing, and may require more severe and unremitting attention on your part, than the greater number of the inquiries which await us, it is still more important than any of these, because it is, in truth, essential to them all. The season of your life is not that which gathers the harvest; it is that which prepares the soil, by diligent cultivation, for the fruits which are to adorn and enrich it ;or, to speak without a metaphor, you do not come here, that you may make yourselves acquainted, in a few months, with all the phenomena of the universe, as if it were only to look on the motions of the planets in an orrery, or to learn a few names of substances and qualities, but that you may acquire those philosophical principles, which in the course of a long and honourable life, are to enable you to render yourselves more familiar day with the works of nature, and with the sublime plans of its beneficent Author:-and if without the knowledge of a single word of fact, in matter or in mind, it were possible for you to carry away from these walls a clear notion of the objects of inquiry, and of the plan on which alone investigation can be pursued with advantage, I should conceive, that you had profited far more, than if, with confused notions of the objects and plan of investigation, you carried with you the power of talking fluently of observations, and experiments, and hypotheses, and systems, and of using, in their proper places, all the hardest words of science. every I must remark, however, that I should not have thought it necessary, thus to direct so much of your attention to the principles of scientific inquiry in general, if I could have taken for granted, that you had already enjoyed the benefit of the instruction of my illustrious colleague in another chair, whose Lectures on Natural Philosophy, exemplifying that soundness of inquiry, which I can only recommend, would, in that case, have enlightened you more, as to the principles of physical investigation, than any mere rules, of which it is possible to point out to you the utility and the excellence. All physical science, whatever may be the variety of objects, mental or material, to which it is directed, is nothing more than the comparison of phenomena, and the discovery of their agreement or disagreement, or order of succession. It is on observation, therefore, or on consciousness, which is only another name for internal observation, that the whole of science is founded; because there can be no comparison, without observation of the phenomena compared, and no discovery of agreement or disagreement, without comparison. So far, then, as man has observed the phenomena of matter or of mind, so far, and no farther, may he infer, with confidence, the properties of matter and of mind; or, in the words of the great primary aphorism of Lord Bacon, which has been so often quoted, and so often quoted in vain, "Homo, naturæ minister et interpres, tantum facit et intelligit, quantum de naturæ ordine re vel mente observaverit ; nec amplius scit aut potest. What is it that we truly mean, however, when we say, that we are about to inquire into the nature and properties of any substance? The question is a most important one, and is far from being so simple as it may at first appear. From the mere misunderstanding of the import of this question, the brightest talents of a long succession of ages, talents, which, with clearer views of this single point, might have anticipated all the discoveries of our own time, and introduced us, perhaps, to discoveries still more brilliant and astonishing, were wasted in inquiries as barren as the frivolous glory which attended them,-that produced indeed much contention, and more pride, but produced nothing more; and, without giving any additional knowledge, took away from ignorance only its humility, and its power of being instructed. What is it that we truly have in view, or should have in view, when we inquire into the nature of a substance? The material universe, and all the separate substances which compose it, may be considered in two lights,-either simply, as composed of parts that coexist, and are to our feelings continuous, so as to form, of many separate and independent elements, one apparent whole; or of parts that change their relative positions, constituting, by this change of place, all the physical events of the material system of the world; and inquiry may have reference to a substance in both, or either of those points of view. What is this body? may be inquired of us, when any particular body is pointed out; and the answer which we give will be very different according to the particular light in which we may have viewed it, though it must always relate to it in one or other of these two aspects. Let us suppose, for example, the body, concerning which the question is put, to be a piece of glass; I select intentionally a substance which is familiar to you all, and of which many of you probably have sufficient chemical knowledge to be acquainted with the * Nov. Org. Aph. 1. composition. It may be asked of us, then, What is the substance termed glass? and our answer will vary, as I have said, with the view which we take of it. If we consider it merely as a continuous whole, our answer will be, that it is a compound of alkaline and siliceous matter-meaning that particles of alkali and flint coexist, and are apparently continuous, in that mass of which we speak. It Such is one of the answers which may be given to the question; and this sort of answer is one which is very commonly given to such questions. is, you will perceive, nothing more than the enumeration of the constituent parts of the substance, and considers the substance, simply as it exists alone, without regard to any other bodies that may exist around it, or near it, and without any allusion to change of any kind. This sort of view, however, may be altogether reversed; and, instead of thinking of the parts that exist together in the substance, without reference to any changes, of which it is either the agent or the subject, we may think only of such changes, without reference to its constituent parts. In this latter point of view, we may say, in answer to the question, as to the nature of the substance termed glass, that it is a transparent substance, which, according to the general laws of refraction, bends the light that passes through it variously, according to the different density of the medium through which the rays have immediately passed before arriving at it, or of the medium, through which they are to pass after penetrating it; that it is a substance fusible at a certain temperature, not dissolved by the common powerful acids, but soluble in a particular acid termed the fluoric acid; that, when strongly rubbed, by certain other substances, it communicates, for a time, to various bodies, the power of attracting or repelling other bodies; and we may add to our description, in like manner, as many other qualities as there are various substances which produce in it any change, or are in any way changed by it. In all answers of this kind, you will perceive that regard is uniformly had, not to the mere substance, concerning which the question is put, but also to some other substance with which, in consequence of some motion of one or other of the bodies, at the time of the phenomenon of which we speak, it has changed its relative position; for, if all the objects in nature remained constantly at rest, it is very evident that we could have no notion of any property of matter whatever. In the enumeration of the qualities of glass, for example, when we speak of its properties, we suppose it to have changed, in every case, some relative position with the light that passes through it, the heat that melts it, the fluoric acid that dissolves it, and the various bodies that excite in it, or conduct from it, electricity; and all these bodies, therefore, we must have in view, in our enumeration, as much as the glass itself. As there are only these two different aspects in which matter can be viewed, all the physical inquiry, with respect to matter, must, as I have said, have reference to one of them; and if we think that we are inquiring further concerning it, our inquiry is truly without an object, and we know not what we seek. We may consider it, simply as it exists in space, or as it exists in time. Any substance, considered as it exists in space, is the mere name which ourselves give to the coexistence of a multitude of bodies, similar in nature, or dissimilar, in apparent continuity; considered as it exists in time, it is that which is affected by the prior changes of other bodies, or which itself produces a change of some sort in other bodies. As it exists in space, 1 therefore, we inquire into its composition, or, in other words, endeavour to discover what are the elementary bodies that coexist in the space which it occupies, and that are all which we truly consider, when we think that we are considering the compound as one distinct body. As it exists in time, we inquire into its susceptibilities or its powers, or, in other words, endeavour to trace all the series of prior and subsequent changes, of which its presence forms an intermediate link. This, then, is our meaning, when we speak of inquiring into the nature of a substance. We have one, or both of two objects in view, the discovery of the separate bodies that coexist in the substance, or rather that constitute the substance, which is nothing more than the separate bodies themselves; or the discovery of that series of changes, of which the presence of this particular substance, in some new relative position with respect to other bodies, forms a part; the changes which other bodies, in consequence of this altered relative position, occasion in it, with the changes which it occasions in other bodies. On these two different objects of physical investigation, the coexisting elements of bodies, and their successions of changes, it may be of advantage to dwell a little more fully in elucidation of the method which we have to pursue in our own department of physical research; for, though it may perhaps at first appear to you, that to treat of the principles of inquiry, in the physics of matter, is to wander from the intellectual and moral speculations which peculiarly concern us; it is in truth only as they are illustrative of inquiries which we are to pursue in the physiology of the mind, that I am led to make these general remarks. The principles of philosophic investigation are, as I have already said, common to all the sciences. By acquiring more precise notions of the objects of any one of them, we can scarcely fail to acquire, in some degree, more precision in our notions of every other, and each science may thus be said to profit indirectly by every additional light that is thrown upon each. It is by this diffusive tendency of its spirit, almost as much as by its own sublime truths, and the important applications of these to general physics, that the study of geometry has been of such inestimable advantage to science. Those precise definitions which insure to every word the same exact signification, in the mind of every one who hears it pronounced, and that lucid progress in the developement of truth after truth, which gives, even to ordinary powers, almost the same facility of comprehension with the highest genius, are unquestionably of the utmost benefit to the mathematical student, while he is prosecuting his particular study, without any contemplation of other advantages to be reaped from them. But there can be no doubt that they are, at the same time, preparing his mind for excellence in other inquiries, of which he has then no conception; that he will ever after be less ready to employ, and be more quicksighted than he would otherwise have been in detecting vague and indefinite phraseology, and loose and incoherent reasoning; and that a general spirit of exactness and perspicuity may thus at length be diffused in society, which will extend its influence, not to the sciences merely, but, in some faint degree, also to works of elegant literature, and even to the still lighter graces of conversation itself. "The spirit of geometrical inquiry," says Fontenelle, "is not so exclusively attached to geometry, as to be incapable of being applied to other branches of knowledge. A work of morals, of politics, of criticism, or even of eloquence, will, if all other circumstances have been |