Unit

A unit is an existent regarded as a separate member of a group of two or more similar members. In Objectivism the term names the specific way of looking at things that makes concept formation possible, and Ayn Rand called it the bridge between metaphysics and epistemology.

“A unit is an existent regarded as a separate member of a group of two or more similar members.”
~ Ayn Rand, "Cognition and Measurement", Introduction to Objectivist Epistemology

What the term adds to "existent"

An existent is whatever is: a thing, an attribute, an action. It is what it is independently of anyone's awareness of it.

"Unit" is not a second kind of thing standing beside existents. It is an existent considered in a certain relationship — as one of a group of similar things, with the similarity noticed and the thing treated as interchangeable with the others for the purpose at hand. That relationship is real, but it takes a consciousness to regard anything under it.

The three pens on a desk are three existents whether or not anyone is looking.
Treating them as three pens — as members of a group whose differences are being set aside — is an act of a mind.
The similarity is not invented. The pens really do resemble each other more than any of them resembles the desk.

This is why Rand called the concept a bridge. Units do not exist qua units; what exists are things. But units are not arbitrary either, because the relationship a consciousness picks out is one the things actually stand in. See Objectivity and Existents.

Why it matters

The unit-perspective is what lets a mind escape the limits of direct awareness. A consciousness that could only hold particulars would be confined to what it can perceive and remember at a given moment — the limit Rand discussed under the heading of the crow-epistemology, after experiments suggesting crows cannot discriminate quantities past a very small number.

Regarding things as units breaks that ceiling. Once "pen" names a group rather than an itemised list, a person can think about pens he has never seen, count them, plan for them and reason about them without holding any particular one in mind. Rand held that the ability to regard entities as units is man's distinctive method of cognition, and the root of the unit-economy that concepts provide.

Units are also what makes measurement possible, since to measure is to relate a quantity to a standard unit of the same kind of attribute. The connection runs both ways: measurement presupposes units, and measurement omission is what turns a group of units into a concept.

Common misunderstandings

That a unit is a mental construct with no basis in fact. The similarity is in the things. What the mind contributes is the act of noticing it and treating the differences as irrelevant for a purpose. Denying the first half gives nominalism; denying the second gives intrinsicism.
That "unit" means a unit of measurement. A foot or a gram is one application of the idea, not its meaning. Any existent regarded as one of a similar group is a unit in this sense.
That units must be identical. They must be similar — possessing the same characteristic in differing measure or degree. Two pens of different lengths and colours are units of the same concept precisely because the differences are quantitative.
That a group of one can have units. The definition requires two or more, because there is nothing to be similar to otherwise. A concept subsuming a single existent, such as a proper name, works differently.

Discussion

References

Rand, Ayn. "Cognition and Measurement", Introduction to Objectivist Epistemology. The Objectivist, 1967.
Peikoff, Leonard. Objectivism: The Philosophy of Ayn Rand, chapter 3.

External links

Unit at the Ayn Rand Lexicon

Epistemology Topics
Senses \| Consciousness \| Volition \| Concepts: Unit, Concept-Formation
Objectivity \| Knowledge: Context, Hierarchy \| Reason: Certainty, Truth, the Arbitrary \| Emotions