Empiricism

Empiricism is the epistemological method of staying at the perceptual level — collecting concretes and particular facts while distrusting the abstractions that would organise them. Objectivism treats it as the mirror image of rationalism: where the rationalist keeps his abstractions and loses reality, the empiricist keeps reality and loses the means of understanding it.

The narrow and the broad sense

In the history of philosophy, empiricism names the school holding that all knowledge derives from sense experience. In that broad sense Objectivism agrees: there is no innate knowledge, and every concept is ultimately traceable to the perceptually given. See Perception and Senses.

What Objectivism rejects is the strict form, which draws a further conclusion — that since only particulars are perceived, only particulars are real, and universals are therefore names, conventions or habits of association rather than identifications of anything. That position is nominalism, and it converts the valid starting point into a denial of conceptual knowledge. See Concept and Introduction to Objectivist Epistemology.

How the method works

Facts are accumulated without integration. Data is gathered, and the question of what it means is deferred indefinitely, since any answer would require an abstraction the empiricist does not trust.
Principles are treated as suspect. A generalisation is regarded as an overreach beyond the evidence rather than as the identification the evidence supports. See Induction.
Every case is a new case. Lacking principles, the empiricist cannot recognise a situation as an instance of anything, so he must handle each one from scratch — and in practice, by whatever worked last time.
Certainty is downgraded to probability. If no generalisation is ever fully warranted, all conclusions become tentative in principle, which leads directly to skepticism.

The practical expression of this method in ethics and politics is pragmatism: action guided by immediate consequences because principles have been ruled unavailable.

Why the two errors converge

Rationalism and empiricism look like opposites and are usually presented as the two available options. Objectivism holds that they share a premise: that abstractions and concretes are not connected — that a concept is either a self-sufficient object of thought or a mere name for a heap of particulars.

Grant the split and each side is left with half of cognition:

The rationalist takes the abstractions and treats them as needing no base, arriving at systems about nothing.
The empiricist takes the concretes and treats abstraction as illegitimate, arriving at data about nothing in particular.

They also produce each other. A rationalist who cannot apply his principles to a case ends up handling cases ad hoc; an empiricist who needs some way to organise his data ends up borrowing an unexamined framework. See Analytic-synthetic dichotomy, which Peikoff argued is the same split stated as a theory of propositions.

Objectivism's answer is that a concept is neither of these things. It is a mental integration of concretes, formed by measurement omission, which retains its referents rather than replacing or abandoning them — which is what makes an abstraction about the particulars it came from. See Objectivity and Hierarchy.

Common misunderstandings

That Objectivism is anti-empirical. It holds that all knowledge rests on the evidence of the senses and that no proposition is exempt from that requirement. The disagreement is over whether abstraction adds knowledge or subtracts it.
That it means being practical or fact-focused. Attending to facts is a virtue. The error is treating the facts as unable to support a conclusion wider than themselves.
That the sciences are empiricist. Science is a search for principles — for causal laws with unlimited scope — which strict empiricism cannot account for. See Science and Causality.
That "just look at the evidence" settles a dispute. Which evidence is relevant is itself a conceptual question, answered by the framework a person is already using. The empiricist has one; he simply has not examined it.

Discussion

References

Rand, Ayn. Introduction to Objectivist Epistemology. 2nd edition, Meridian, 1990.
Peikoff, Leonard. "The Analytic-Synthetic Dichotomy", in the same volume.
Peikoff, Leonard. Objectivism: The Philosophy of Ayn Rand, chapters 3 and 5.