unknown quantity
A person or thing whose nature or value is a mystery.
Whatever he had learned of himself was here of no avail. He was an unknown quantity.
noun
A fundamental, generic term used when referring to the measurement (count, amount) of a scalar, vector, number of items or to some other way of denominating the value of a collection or group of items.
You have to choose between quantity and quality.
A few comments on the words used in the title of the book and on some related expressions are necessary. A "quantity" is here understood in the sense of "a thing that has the property of being measurable in dimensions, amounts, etc., or in extensions of these which can be expressed in numbers and symbols" (Webster's New World Dictionary of [the American Language], Third Edition, 1988), or of "something having magnitude, or size, extent, amount, and the like" (Random House Webster's College Dictionary, 1991; The New Hamlyn Encyclopaedic World Dictionary, 1988). It is interesting that up to the 1950s a "quantity" was defined as what we now describe as "magnitude" (e.g., the unabridged Webster's New International Dictionary, 1948, gives examples of a sphere's surface being a quantity, its area a magnitude, or a yardstick being a quantity, its length a magnitude). This practice is now obsolete and at present length is a quantity, its numerical value its magnitude.
An indefinite amount of something.
Some soap making oils are best as base oils, used in a larger quantity in the soap, while other oils are best added in a small quantity.
Olive oil can be used practically in any quantity.
A specific measured amount.
This bag would normally cost $497.50 for a quantity of 250, at a price of $1.99 per piece.
Generally it should not be used in a quantity larger than 15 percent.
A considerable measure or amount.
The Boeing P-26A was the first all-metal monoplane fighter produced in quantity for the U.S. Army Air Corps.
Property of a phenomenon, body, or substance, where the property has a magnitude that can be expressed as number and a reference.