MIA: Encyclopedia of Marxism: Glossary of Terms
Quality and Quantity
Quality is the basic character or nature of something. Quantity is a variable amount of a thing, where the amount does not affect the quality (the basic nature) of what that thing is.
When the quantity of something changes, i.e. if a book has 100 pages as opposed to 50 pages, a quantitative change has occurred. There are times however, when the amount of change in a thing changes its very character. For example, if the book was reduced to one page, it is no longer a book. When change affects the very character of the thing, a qualitative change has occurred.
Further Reading: In Hegel's Logic, Quality is the first division of Being, when the world is just one thing after another, so to speak, while Quantity is the second division, where perception has progressed to the point of recognising what is stable within the ups and downs of things. The third and final stage, Measure, the unity of quality and quantity, denotes the knowledge of just when quantitative change becomes qualitative change.
Quantity and Quality, Law of Transformation
Hegel presents the relation between Quantity and Quality as follows:
The critical limit between finite and infinite;
Water turning into ice etc - "nodal lines";
transition from quantity into quality ... based in a difference of magnitude;
Measure as the Unity of Quantity and Quality;
Quantity just means quality superseded and absorbed;
Measure in Nature and Society;
Quantity and Quality in Chemistry.
Further Reading: section in Hegel's Outline of Logic.
See also: the Letter from Marx to Engels, 22 June 1867, in which Marx formaulates the Law of transformation of Quantity into Quality and attributes it to Hegel in the Logic.