The starting point of Objectivist Epistemology is the principle, presented by Rand as a direct consequence of the metaphysical axiom that “Existence is Identity,” that Knowledge is Identification. Objectivist epistemology studies how one can translate perception, i. e. , awareness acquired through the senses, into valid concepts that actually identify the facts of reality. Objectivism states that by the method of reason man can gain knowledge (identification of the facts of reality) and rejects philosophical skepticism.
Objectivism also rejects faith and “feeling” as means of attaining knowledge. Although Rand acknowledged the importance of emotion in humans, she maintained that the existence of emotion was part of our reality, not a separate means of achieving awareness of reality. Rand was neither a classical empiricist (like Hume or the logical positivists) nor a classical rationalist (like Plato, Descartes, or Frege). She disagreed with the empiricists mainly in that she considered perception to be simply sensation extended over time, limiting the scope of perception to automatic, pre-cognitive awareness.
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Thus, she categorized so-called “perceptual illusions” as errors in cognitive interpretation due to complexity of perceptual data. She held that objective identification of the values of attributes of existents is obtained by measurement, broadly defined as procedures whose perceptual component, the comparison of the attribute’s value to a standard, is so simple that an error in the resulting identification is not possible given a focused mind.
Therefore, according to Rand, knowledge obtained by measurement (the fact that an entity has the measured attribute, and the value of this attribute relative to the standard) is “contextually certain. ” Ayn Rand’s most distinctive contribution in epistemology is her theory that concepts are properly formed by measurement omission. Objectivism distinguishes valid concepts from poorly formed concepts, which Rand calls “anti-concepts. ” While we can know that something exists by perception, we can only identify what exists by measurement and logic, which are necessary to turn percepts into valid concepts.
Procedural logic (defined by Rand as “the art of non-contradictory identification”) specifies that a valid concept is formed by omitting the variable measurements of the values of corresponding attributes of a set of instances or units, but keeping the list of shared attributes – a template with measurements omitted – as the criterion of membership in the conceptual class. When the fact that a unit has all the attributes on this list has been verified by measurement, then that unit is known with contextual certainty to be a unit of the given concept. 9] Because a concept is only known to be valid within the range of the measurements by which it was validated, it is an error to assume that a concept is valid outside this range, which is its (contextual) scope. It is also an error to assume that a proposition is known to be valid outside the scope of its concepts, or that the conclusion of a syllogism is known to be valid outside the scope of its premises. Rand ascribed scope violation errors in logic to epistemological intrinsicism. 
Rand did not consider the analytic-synthetic distinction, including the view that there are “truths in virtue of meaning,” or that “necessary truths” and mathematical truths are best understood as “truths in virtue of meaning,” to have merit. She similarly denied the existence of a priori knowledge. Rand also considered her ideas distinct from foundationalism, naive realism about perception like Aristotle, or representationalism (i. e. , an indirect realist who believes in a “veil of ideas”) like Descartes or Locke.
Objectivist epistemology, like most other philosophical branches of Objectivism, was first presented by Rand in Atlas Shrugged.  It is more fully developed in Rand’s 1967 Introduction to Objectivist Epistemology.  Rand considered her epistemology and its basis in reason so central to her philosophy that she remarked, “I am not primarily an advocate of capitalism, but of egoism; and I am not primarily an advocate of egoism, but of reason. If one recognizes the supremacy of reason and applies it consistently, all the rest follows. “