Logic recognizes a principal division in class names, according as these are the names of objects which agree with each other and ...
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Logic recognizes a principal division in class names, according as these are the names of objects which agree with each other and differ from other objects in a very large and indefinite number of particulars or attributes, or are the names of objects which agree only in a few and a definite number of attributes. The former are the names of "real kinds," and include the names of natural species, as man, horse, etc., and of natural genera, as whale, oak, etc.... For examples of names that are not the names of "real kinds," we may instance such objects as those that are an inch in length, or in breath, or are colored black, or are square, or (combining these particulars) such objects as black square inches.

