The title pretty much says it all, but let me elaborate on the source of confusion. VIM defines quantity as: Other definitions I found are extremely circular. They typically go like this, "a physical quantity is a property that can be quantified". In other words, they explain quantity with quantity. When you add the fact that a unit of measurement is a quantity itself, no definition seems explanatory! So far, my understanding is that properties that can be measured (e.g. weight, length, etc.) are referred to as quantities. More subjective properties like taste and smell are not quantities, since they can't be compared to any accepted standard. Would it be safe to say that psychologists generally deal with properties, while physicists with quantities? To summarize, are quantities assignable to properties or is a quantity a property in its own right? A clear separation between the terms you use would be much appreciated. Thank you!