Floating Abstraction

A floating abstraction is a concept a person uses without knowing what concretes it stands for — an abstraction taken over from other people and detached from the perceptual evidence that would give it meaning. The term is Ayn Rand's, and the error it names is a failure to complete concept formation rather than a mistake in reasoning from concepts already formed.

“Floating abstractions are concepts detached from existents, concepts that a person takes over from other men without knowing what specific units the concepts denote.”
~ Ayn Rand, "The Psycho-Epistemology of Art", The Romantic Manifesto

How a concept comes loose

A valid concept is formed by isolating concretes that share an attribute and omitting their particular measurements; its meaning is the units it subsumes. See Introduction to Objectivist Epistemology and Measurement omission.

Words, however, can be learned without that process ever being performed. A person hears "justice", "capitalism", "inflation" or "rights" used in sentences, learns which sentences sound right, and acquires the ability to use the word fluently — while never having identified what in reality the word refers to. What he holds is a verbal token with the shape of knowledge and none of the content.

The mark of a floating abstraction is that it cannot survive the question "such as?" A person who can define capitalism but cannot name a concrete case, or say what would distinguish one from a non-case, is holding the word rather than the concept.

Why it matters

It is undetectable from the inside. A floating abstraction feels exactly like knowledge. The person can use the word in argument, agree and disagree with others about it, and never encounter the gap.
It cannot be corrected by evidence. Since the concept was never tied to concretes, no concrete can contradict it. This is what makes floating abstractions so durable in political and moral debate.
It breaks the hierarchy. Conceptual knowledge is built in layers, each resting on the ones below. An abstraction with no base does not merely fail on its own; everything a person builds on top of it floats too.
It is the raw material of rationalism. Deducing elaborate systems from terms whose referents were never identified is how a person can be internally consistent and completely detached from reality at once.

The corrective Rand prescribed is reduction: tracing a concept back down the hierarchy, level by level, to the perceptual concretes it ultimately rests on. See Concept and Definitions.

Related errors

Frozen abstraction — substituting one particular concrete for the wider abstraction that subsumes it, as in treating altruism as though it were the whole of morality.
Package deal — packing together things that do not belong under one concept, so that accepting the word smuggles in the association.
Invalid concept — a word that could not name a legitimate concept at all, because the alleged units have no common attribute.
Stolen concept — using a concept while denying the more fundamental ones it depends on.

Common misunderstandings

That any abstract term is a floating abstraction. High-level abstraction is how conceptual knowledge works. The charge concerns concepts held without their referents, not concepts that are far from the perceptual level.
That it means being unable to define a word. Many people use valid concepts correctly without being able to formulate a definition. The test is whether they can identify the units, not whether they can produce a formula.
That it is an accusation of stupidity. Rand's point was that the education most people receive teaches words rather than concept formation, so the error is widespread and unchosen. What is chosen is whether to check.

Discussion

References

Rand, Ayn. "The Psycho-Epistemology of Art", The Romantic Manifesto.
Rand, Ayn. Introduction to Objectivist Epistemology. 2nd edition, Meridian, 1990.
Peikoff, Leonard. Objectivism: The Philosophy of Ayn Rand, chapter 4.