By the slow and laborious methods of observation, hypothesis, deduction and experimental verification, a scheme has been constructed which for the most part is consistent with itself, and bears the test of the comparison of one part with another. As a chart is drawn by the explorer of unknown seas to represent his discoveries in a conventional manner, so the scientific investigator constructs a mental model of the phenomena he observes, and tests its consistency with itself and its concordance with the results of further experiment. The chart does not give a lifelike picture of the coast as does a painting, but it represents one aspect of it conventionally in a manner best adapted for the immediate pur pose. So the conceptions of one branch of science—mechanics, let us say—represent the phenomena of nature in the conventional aspect best suited for one particular line of inquiry. It does not follow necessarily that "nature" in reality resembles the particular mental chart which mechanical science enables us to construct. It does not even follow that there is any "reality" underlying phenomena and corresponding with any of our conceptions. The whole problem which mankind has to face undoubtedly includes an inquiry into the ultimate nature of reality. But that inquiry lies in the province of metaphysics, and is not necessarily involved in the pursuit of natural science. Metaphysics uses the results of natural science, as of all other branches of learning, as evidence bearing on her own deeper and more difficult questions. But it does not follow that natural science must solve metaphysical prob lems before being of use to man and enlarging the sphere of his knowledge. We need not ask whether the reality is represented accurately by our conventional model before using that model to introduce order into what would otherwise be mental confusion, and to enable us to make systematic and progressive use of natural resources. It is true that the possibility of constructing consistent schemes of scientific concepts is an argument in favour of the exist ence of a definite reality underlying phenomena resembling in some respects the pictures of it we draw. But metaphysicians are not agreed that it is a conclusive argument.
The difficulty of making a scientific picture of the ultimate nature of reality may be illustrated by an example. Our first con ception of a wooden stick involves the ideas of a certain long shaped form, of smoothness, of hardness, of weight, of a certain brown colour, perhaps of some amount of elasticity. A microscope reveals a structure much more detailed than we imagined, and our mental model of the stick ceases to be smooth. It becomes co ordinated with those of a number of other bodies which we know to be parts of trees, the growth and structure of which we study by the help of botany. From the results of observation and experiment, physics teaches us that the properties of the stick can only be represented satisfactorily by imagining that the sub stance of it is not infinitely divisible, that it consists of discontinu ous particles or molecules. Again, chemistry assures us that the molecules of the stick are made up of still smaller parts or atoms, which separate from each other when, for instance, the stick is burned, and afterwards can arrange themselves into new molecules.
When we pursue our inquiries into the nature of these atoms, we find that they can be resolved, partly at any rate, into much smaller particles or corpuscles in continual motion within the atom. These corpuscles themselves have been identified with isolated units of negative electricity or electrons, the vibrations of which within the atom sort out the electromagnetic radiation which falls on them and allow to reach our eyes those waves only which give us the sensation of brown colour.
At present pioneers are attempting to look beyond the electron into still more intimate details of structure. But we have travelled far from our original conception of the nature of the stick, and, should the problem last stated be solved, we should only find our selves faced by the next one, the nature of the units at which, for the time being, we have arrived. But what constitutes reality? Where, in the endless chain of explanations discovered or to be discovered, can we stop and say : "Here is the true picture of what the stick is"? But this impossibility does not prevent us from get ting the full use of each conception in turn when used for its par ticular purpose. To the schoolboy, the effective and deterrent con ception of the stick is that of a hard, elastic, long-shaped solid. The botanist regards it as built up by the action of vegetable cells, which he refers to a particular kind of tree. To the chemist the stick is made up of atoms of carbon, hydrogen and oxygen, each with definite properties and arranged in certain combinations. The physicist sees these atoms composed of whirling electrons, each an ultimate electric unit not yet capable of further expla nation. Each idea is useful in turn. Since science is analytic it tends to regard the physical concepts, such as space, time, mass, atom, electron, as the most fundamental. They may or may not correspond with something in ultimate reality : with that science is not concerned. But accepting as convenient these and other less fundamental concepts, science examines experimentally the rela tions between them, and it is these relations which are the Laws of Nature about which so much has been said. We may not know what realities, if any, underlie the concepts of mass, length or time, of electric current or electro-motive force. But Ohm found that whatever they really be, current is proportional to electro-motive force, and Newton showed that, to a very high order of accuracy, two masses produce a mutual acceleration pro portional directly to the product of the masses and inversely to the square of the distance between them. Nowadays we believe that mass, time and space are not what Newton thought, and the exact point of accuracy at which his law fails has been discovered by Einstein. But the law remains as a permanent achievement, all the more useful because its limits of accuracy are known. The gradual piecing together of such natural laws constitutes science, and the ultimate test of its validity can only be the final con sistency with each other of the parts of the whole structure.