LOGIC, the art of reasoning. As the necessities of our existence oblige us to think, and to arrange eur thoughts in such a manner as may enable us to com municate with each other, we are habitu ally impelled towards a conclusion, that it is unnecessary to teach reasoning as an art. It is hardly needful to combat this notion by arguments which will easily occur to most men of reflection ; and indeed the contrary persuasion was so prevalent in the middle ages, that men seem to have been more occupied with the art, than with the pro per use of it.
In order to reason well, it is necessary that the nature of our perceptions and ideas, and the notions or conclusions we draw from them, should be well under stood. Logic, therefore, is a science of extensive occupation ; which has its be ginning in the constitution of things, and the processes of the human intellect, and its practical termination in the structure, use, and application of language. Its objects are no less than the universal acquisition of knowledge, and that mu tual communication wriich constitutes a large part of the employment, and is the most distinguishing character of man.
The impressions made by external ob jects upon the senses, are called sensa tions or ideas of sensation. See Inzot GY. The recollection or remembrance of those sensations are simply called ideas. The general notions which are produced in the mind by reflecting upon ideas have been called ideas of reflection ; but as they all grow out of the comparison of the first-mentioned ideas, and do univer sally in the last result imply propositions, it appears much preferable to call them notions.
Logical writers divide ideas into simple and complex; but as we have no simple sensations, and can therefore have no sim ple ideas but by the artificial process of abstraction, the division seems useless. The word complex here signifies com pounded, and the compounded nature of our ideas will practically depend, in a great measure, upon our choice or deter mination in the subject of our reasoning.
Thus, a lemon is soft, fragrant, yellow, and acid. If I throw a lemon at another, the attention will be chiefly directed to the organ of touch, and its fragrance, its tint, and its acidity, will be abstracted or left out. But the perfumer, the designer, and the chemist, would separately attend to those parts of the idea which were sug gested by the organs of smell, of vision, and of taste. And in this manner it is that we may separate the simple ideas of yellowness, acidity, and fragrance; though, in nature, their causes never appear him lated and apart from those of all the other sensations.
Abstraction, or the leaving out parts of ideas or notions ; generalization, or class ing things together, as possessing the re. maining distinctive characters ; composi tion, or the re-assumption of some of the abstracted or rejected ideas, are the vo luntary acts of the mind, adopted in order to facilitate the useful process of com parison. Thus we may abstract from bo. dies all ideas but those of structure, and divide them into organized and unorganiz ed; or we may take the organized bodies, and call them animals and vegetables ; or we may attend to their place of existence, and call them terrestrial, aquatic, volatile, and the like ; and many of our most use ful propositions wilt thus, in all our men tal operations, continue equally general and abstracted.
In the scientific arrangement of natural objects, philosophers have pursued the course of abstraction, until, by rejecting all the ideas capable of affording the dis tinctive characters of individuals, they ar rived at an hypothetical being called sub stance. Much has been written concern ing it ; but it will perhaps be attended with the least obscurity to say, that it is supposed to be an independent existence, which serves as the basis or support to those properties which are perceived by our senses ; or, in the words of lo gicians, it is the subject of modes and ac cidents.