|Ayn Rand and Objectivism:
A handful of other philosophies besides Objectivism have defended perceptual realism, the theory that we directly perceive things in the external world. But Objectivism is the first philosophy to identify the connection between the primacy of consciousness and the various false theories of the relation between consciousness and existence, among them: representationalism, phenomenalism, idealism, the "sense-data" theory, and others. All these false theories get "sucked in" by this argument:
Sound illogical? It is, but that hasn't stopped countless philosophers from denying that we perceive reality. See what happens when you deny the primacy of existence? You assume that the content of consciousness (what's in your head) is the first thing you know for certain, instead of what's in reality. Then you're stuck in a futile search for some assurance that the things you think are real aren't delusions, fantasies, or elaborate hoaxes.
Ayn Rand also defended a unique theory of concepts, or abstract ideas. For her, the ability to form concepts was the primary function of the faculty of reason, the possession of which is the essential difference that sets humans apart from all the other animals. Rand presented her theory of concepts as a solution to the legendary "problem of universals," rekindling interest in a problem that essentially died in the late medieval era.
The problem of universals is basically this: we know that any two people (or cars, or trees, or triangles, or whatever) are different, but how is that we all know what we mean when we say that that two things which are clearly different are also of the same kind or type of thing? What is exemplified "universally," i.e., by all men (cars, trees, triangles ...)? The two main historical alternatives on this issue are "nominalism" and "classical realism."
Nominalism says that no two things are the same, and that we know what each other means because there are social conventions in place that govern how we do things with words. The notion that there could be literally abstract ideas is supposed to be incoherent. Classification is entirely pragmatic.
Classical realism says that all things have some special inner essence or "form" that justifies calling all things that have the same form by the same name. Concept-acquisition depends on a process much like an intuition or "sixth sense," we are just supposed to grasp the essences of things.
The key to Ayn Rand's alternative solution lies in her theory of concept-formation, and specifically in her identification of measurement-omission, a process explained in detail in Introduction to Objectivist Epistemology.
The Objectivist theory of concepts provides a set of rules for deriving good ("valid") concepts, just as Aristotle provided a set of rules for deriving valid inferences. The Objectivist theory has many significant and interesting implications: the rejection of the analytic-synthetic distinction, and the rejection of the terms in which the modern "problem of induction" is formulated in analytic philosophy of science. It undercuts nominalism at its root by exposing the subjectivism behind it, while it rejects classical realism by showing the pointlessness of the intrinsicist view of essences.
Since the objectivity of our rational knowledge depends on (i) the validity of our reasonings, (ii) the soundness of our definitions, and (iii) the validity of our concepts, Objectivism advances the project of the Enlightenment by forging the critical theoretical link between ideas and reality. Rand herself regarded her vindication of the power of reason as her signal achievement, and named her philosophy "Objectivism" in order to register her unflagging confidence in the possibility of objective knowledge of facts and values.
With the power of reason fully vindicated, there is no role for faith in living a good life.