Conceptual Common Denominator
We differentiate objects according to some commensurable characteristic, which Ayn Rand called the conceptual common denominator. In the table/chair example, the conceptual common denominator was shape. The difference in shape between chairs and tables is so much more than the difference between two tables that we view the large difference is qualitative and the small difference as merely quantitative (which we latter elide).
"A commensurable characteristic (such as shape in the case of tables, or hue in the case of colors) is an essential element in the process of concept-formation. I shall designate it as the "Conceptual Common Denominator" and define it as "The characteristics reducible to a unit of measurement, by means of which man differentiates two or more existents from other existents possessing it."
The distinguishing characteristic(s) of a concept represents a specified category of measurements within the "Conceptual Common Denominator" involved.
New concepts can be formed by integrating earlier-formed concepts into wider categories, or by subdividing them into narrower categories (a process which we shall discuss later). But all concepts are ultimately reducible to their base in perceptual entities, which are the base (the given) of man's cognitive development." - "Introduction to Objectivist Epistemology", Ayn Rand
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http://rous.redbarn.org/objectivism/Writing/RaymieStata/epist/subsection3_5_2.html http://originresearch.com/documents/rand6.cfm