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by the objects treated in the Science. The science of astronomy investigates the uniform or similar way in which the heavenly bodies, and in fact all material substances, tend to fall towards each other as a stone falls towards the earth, or to move round each other under the influence of this tendency. The universal law of gravitation is thus the natural law or uniformity treated in physical astronomy.
In chemistry the law of equivalent proportions describes the well ascertained fact that each chemical substance enters into combination with every other chemical substance only in certain definite proportions ; as when exactly eight parts by weight of oxygen unite with one part of hydrogen to form water, or sixteen parts of oxygen and six parts of carbon unite to form carbonic acid in the ordinary burning of a flame or fire. When ever we can detect uniformities or similarities we so far create science and arrive at natural laws. But there may be, and are, many things so fickle, complicated, and uncertain, that we can never be sure we have detected laws that they will uniformly obey; in such cases no science, in the proper sense of the word, is possible. There is no such thing, for instance, as a real science of human character, because the human mind is too variable and complicated a subject of investigation. There are no two persons so much alike that you may be sure of one acting in all circumstances as the other would; it thus becomes impossible to arrange persons in classes so that all who are in the same class shall act uniformly in the same manner in any given circumstances.
But there is a science of human reason or thought apart from the many other acts of mind which belong to human character, because there are modes in which all persons do uniformly think and reason, and must think and reason. Thus if two things are identical with a third
common thing they are identical with each other. This is a law of thought of a very simple and obvious character, and we may observe concerning it,
1. That all people think in accordance with it, and agree that they do so as soon as they understand its meaning.
That they think in accordance with it whatever may be the subject about which they are thinking. Thus if the things considered are
The most populous city in Great Britain, since “the Metropolis is identical with London," and “ London is identical with the most populous city in Great Britain," it follows necessarily in all minds that “the metropolis is identical with the most populous city in Great Britain." Again, if we compare the three following things
The cheapest metal, and it be allowed that “The most useful metal is Iron," and “Iron is the cheapest metal,” it follows necessarily in all minds that “the most useful metal is the cheapest.” We here have two examples of the general truth that things identical with the same thing are identical with each other; and this we may say is a general or necessary form of thought and reasoning. Compare, again, the following three things,
Bodies revolving in elliptic orbits. We cannot say, as before, that “the earth is identical with the planets;" it is identical only with one of the
planets, and we therefore say that "it is a planet.” Similarly we may say that “the planets are bodies revolving in elliptic orbits,” but only a part of the whole number so revolving. Nevertheless it follows that if the earth is among the planets, and the planets among bodies revolving in elliptic orbits, then the earth is among the latter.
A very elementary knowledge of chemistry enables us to argue similarly concerning the following ;
Elementary substances. Iron is one of the metals, and metals are elements or simple undecomposable substances, in the sense of being among them or a part of them, but not as composing the whole. It follows necessarily that “Iron is one of the elementary substances.” We have had then two examples of a fixed and necessary form of thought which is necessary and true whatever the things may be to which it is applied. The form of argument may be expressed in several different ways, and we shall have to consider it minutely in the lessons on the syllogism; we may express it, for instance, by saying that “part of a part is part of the whole.” Iron is part of the class of metals, which is part of the class of elements: hence iron is part of the class of elements.
If I now introduce another definition of Logic and say that it is “the science of the necessary forms of thought,” the reader will I hope clearly apprehend the meaning of the expression “necessary forms of thought.” A form is something which may remain uniform and unaltered, while the matter thrown into that form may be varied. Medals struck from the same dies have exactly the same form, but they may be of various matter, as
bronze, copper, gold or silver. A building of exactly the same form might be constructed either of stone or bricks; furniture of exactly similar shape may be made of oak, mahogany, walnut wood, etc. Just as we thus familiarly recognize the difference of form and substance in common tangible things, so we may observe in Logic, that the form of an argument is one thing, quite distinct from the various subjects or matter which may be treated in that form. We may almost exhibit to the eye the form of reasoning to which belong our two latter arguments, as follows:
(X)..............(2) If within the three pairs of brackets, marked respectively X, Y and Z we place three names, such that the one in place of X may be said to come under that in Y, and that in Y under that in Z, then it necessarily follows that the first (X) comes under the last (Z).
Logic, then, is the science occupied in ascertaining and describing all the general forms of thought which we must employ so long as we reason validly. These forms are very numerous, although the principles on which they are constructed are few and simple. It will hence appear that logic is the most general of all the sciences. Its aid must be more often required than the aid of any other science, because all the particular sciences treat portions only of existing things, and create very different and often unconnected branches of knowledge. But logic treats of those principles and forms of thought which must be employed in every branch of knowledge. It treats of the very origin and foundations of knowledge itself; and though it is true that the logical method employed in one science may differ somewhat from that em
ployed in another science, yet whatever the particular form may be, it must be logical, and must conform to the laws of thought. There is in short something in which all sciences must be similar; to which they must conform so long as they maintain what is true and selfconsistent; and the work of logic is to explain this common basis of all science.
One name which has been given to Logic, namely the Science of Sciences, very aptly describes the all extensive power of logical principles. The cultivators of special branches of knowledge appear to have been fully aware of the allegiance they owe to the highest of the sciences, for they have usually given names implying this allegi
The very name of logic occurs as part of nearly all the names recently adopted for the sciences, which are often vulgarly called the “ologies, but are really the “logics," the “o” being only a connecting vowel or part of the previous word. Thus geology is logic applied to explain the formation of the earth's crust; biology is logic applied to the phenomena of life; psychology is logic applied to the nature of the mind; and the same is the case with physiology, entomology, zoology, teratology, morphology, anthropology, theology, ecclesiology, thalattology, and the rest*. Each science is thus distinctly confessed to be a special logic. The name of logic itself is derived from the common Greek word dóyos, which usually means word, or the sign and outward manifestation of any inward thought. But the same word was also used to denote the inward thought or reasoning of which words are the expression, and it is thus probably that later Greek writers on reasoning were led to call their science
Except Philology, which is differently formed, and means the love or study of words; the name of this science, if formed upon the same plan, would be logology.