THEORY, THEORY AND PRACTICE. If articles upon the mere meaning of words be admissible, it is the consequence of the manner in which the words are used. Of all the fallacies which infest society, the most common is Gist of applying to one sense of a word ideas or associations derived from another; and of all the words in use, there are few which are more often subjected to such process than those which stand at the head of this article.
By theory, properly speaking, is meant the mode of making seen and known the dependence of truths upon one another : a theory is a con nected body of such truths belonging to one or more common principles. The use of this word has enlarged with the boundaries of the sciences. For example, before the discovery of universal gravita tion, all that was known of any one planet was the empirical formulas for one or two of its inequalities. This constituted the theory of the planet (then so called): thus the theory of the moon, eo far as peculiar, consisted in the statement of the laws of the inequalities called the equation of the centre, the evection, d.c. ' In our day the point of view is changed ; it is no longer the mere exhibition of these ine qualities which constitutes the theory, but the deduction of them, as necessary consequences, from the principle of gravitation. The theoretical astronomer now starts from this principle, and, taking only one position and velocity for his numerical data, finds out every inequality of the planetary motions, those which were previously known from observation and more, and shows how to form them into tables. The practical astronomer makes these tables, computes places from them for the current year, compares these places with the results of observation, and returning the comparison into the hands of the theorist, enables him, if need be, to correct the original numerical data to which be applied his methods, or to detect new inequalities. The process is now deductive ; but before the time of Newton it was the other way. The observer had the first task ; the inequalities were to be collected from comparison of observations, and their laws, reduced to the simplest form, were the data for future tables.
Again, before the introduction of the undulatory hypothesis, the theory of light consisted in the exhibition of the laws of reflexion and refraction, with a certain extent of explanation from the emanatory hypothesis of Newton. Since that time the theory of light has become, though at a distance, a resemblance of the theory of gravita tion in its character : prediction has commenced, that is to say, the phenomena which would appear under certain new circumstances have been announced before any experiments were made to discover them ; and correctly announced. This is the end to which theory ought to be
constantly tending ; namely, the discovery of laws of actiou in so complete a manner that the necessary consequences of those laws never fall to make their appearance, so that everything which is seen is found to be a consequence of the laws when examined, and every con sequence of the laws is seen in phenomena when looked for. Whatever fulfils three conditions may be called a perfect theory, or a perfect mathematical theory.
The next step in the chain of discovery is one which may in most cases be incapable of attainment. For example, nothing is more certain than that the assumption of every particle of matter attracting every other particle, according to the Newtonian law, leads to the complete deduction of the celestial motions, and gives the complete power of prediction just alluded to. But whether this ATTRACTION does actually take place, or whether any intermediate agent is employed, though It matters nothing at present to the mathematical theory, is the next object of inquiry. Could this point bo ascertained, it is more than probable that the knowledge of the constitution of matter to which it would lead, would open hundreds of important consequences even in the application of science to the arta. [Cause ; ifvrovnests.] Before coming to the distinction between theory and practice, we must observe that theories may be divided into two classes, the more perfect and the less perfect. Wo cannot say that any theory- is absolutely perfect ; but there are some of which the defects are hardly perceptible, and others in which the contrary is the case. For example, the theory of the statics and dynamics of rigid bodies is tolerably perfect ; but that of bodies composed of particles acted on by molecular forces is in its Infancy. We know a great deal more of the connection of the planetary worlds with each other than we do of the particles which, when connected together, form a bar of iron or of oak. We know that the bar is not perfectly rigid ; that it bends and breaks : and the degree of bending which a given force will cause, and the amount of pressure necessary to produce fracture, must be sought for in experiments, from which, imperfect as they are, the laws which would follow from a good theory, if we had one, are to be deduced.