ABSTRACTION, in metaphysics, a mental opera tion, by which we separate, in thought, things which are naturally conjoined. According to the logicians, .
it is an operation of the mind, by which we detach our conceptions all those circumstances that render them particular, and thereby fit them to denote a whole rank or class of beings. Thus, in coin( mplating the various kinds of triangles, we leave out, by the power of abstraction, every one of their properties, but that of their haling three sides, which thus becomes charac teristic of a whole genus of beings, that may neverthe less differ from each other in many other particulars, such as the length of the it sides, the proportional mag nitude of their angles, &c. In like manner, while consi dering the various individuals of the human race, we discover certain characteristic properties, such as pecu liarities of form, stature, language, reason, &e. which they all possess in common; and which therefore are assigned to the whole of this class of individuals, under the abstract denomination of man. In this manner are formed general ideas, and g, neral terms, the great in struments of scientific research. See LOGIC and ME TAPHYSICS.
Abstraction, according to Mr Locke, may be em ployed in three ways. 1st, the mind may consider some particular part of a thing, distinct front the whole ; as a man's arm, without the consideration of the rest of his body. 2dly, we may consider the mode of a substance, without reference to the substance itself ; or we may separately consider several modes which subsist together in one substance. It is by this kind of abstraction, that geometricians treat separately of the length of bodies, which they call a line, without the consideration of their breadth and thickness. Sdly, by abstraction, the mind frames general, or universal ideas, omitting the modes and relations, which are peculiar to the individual objects whence they are de rived. Thus, when we would understand a thinking being in general, we gather from our own conscious ness, what it is to think ; and, omitting the considera tion of whatever has a peculiar relation to our mind, or to the human mind, we form the conception of a thinking being in general.
'lids last operation of abstraction, by which general ideas are formed, implies the exercise of anothet Inch tal powi•, by w hick we compare together the variou s objects of nature, and discover that one or more attri butes arc CO111111011 to many individuals. This power
has been called generalization ; and it has been said, that though NS C could not generalize without some de gree of abstraction, yet we might abstract without ge I weal izing. Thus, I may attend to the w hi teness of the paper belb•e me, without applying that colour to any other object ; and the whiteness of this ob jeyt is an abstract conception, though not a general one, while applied to one individual only. (See Reid's E.sl.ay.? on ties Intellectual Powers.) This pow e• of generalization appears to be nothing more than an ex ercise of the mental operation more commonly known by the name of association or combination ; es hereby we are led to conjoin in thought objects which resem ble each other in certain particulars, or WhiCil, en en by mere accident, have been presented to the mind tog( the r. By means of this power of combining, and the co-operating faculty of abstraction, we are naturally led to trace the manifold resemblances and differences ss hich the objects of nature exhibit : and by ascertain ing the general laws, to which their various phenomena are subjected, we render them proper subjects for scientific speculation. See Assoc IATION.
The theory of abstract ideas has cost metaphysicians as much trouble as any branch of their abstruse science, and has divided them into sects or parties, between which a keen controversy has long been carried on respecting this subject. Plato and Aristotle, among the ancients, maintained, that abstract ideas are beings which have a real existence, at least in the mind that conceives them : Zeno asserted, that they were mere non-entities. 'Most of the schoohnen sided with At is totle ; but Roscelinus, and Peter Abelard, adopted the opinion or Zeno ; and asserted, that abstract ideas, or 1111iVerS(1h.. as they called them, were nothing but names ; whence they were called nominalists, and their oppo nents got the name of realists. A few chose to place universality, neither in things nor in names, but in ou.r conceptions ; from which they were called concrptitatiste. The disputes on this subject formed the chief, if not the only employment of the learned, during the middle ages ; and sometimes they had recourse to hard blows. when they found themselves deficient in argument.