Senses

The senses are man's faculties of direct awareness of the world: the physical apparatus by which the properties of external objects act on an organism and produce sensory material. In Objectivism the senses are the base of all knowledge, the given from which perception and then concept formation proceed, and their validity is not a conclusion that could be proved from something more basic.

“A percept is a group of sensations automatically retained and integrated by the brain of a living organism, which gives it the ability to be aware, not of single stimuli, but of entities, of things.”
~ Ayn Rand, Introduction to Objectivist Epistemology

Sensation and percept

Objectivism distinguishes two levels below the conceptual.

A sensation is the response to a single stimulus: a patch of colour, a sound, a pressure. It is transient — when the stimulus stops the sensation stops, and it cannot be held in memory or isolated by an adult human, who has been perceiving entities since infancy. A sensation tells the organism that something is there, not what it is.

A percept is a group of sensations automatically integrated by the nervous system into an awareness of an entity. Perception is not a conclusion, an inference or a construction the person performs; it is what the apparatus does, and it happens whether or not he chooses to attend to it. Man is never aware of bare sensations; he opens his eyes and the world is given as objects.

The conceptual level begins where the automatic ends. Concept formation is volitional, and it is the level at which knowledge can be true or false. See Volition and Unit.

Why the senses cannot be "wrong"

The claim that the senses are valid is easily misread as a claim that people never make perceptual mistakes. It is narrower and stronger than that.

Truth and error are properties of identifications, and identification is conceptual. The senses do not identify anything; they deliver material. A faculty that makes no judgment can no more be mistaken than a thermometer can lie. When somebody says the stick in the water "looks bent", the senses have reported exactly what light refracting through water does; what may go wrong is the conclusion drawn about the stick, and it goes wrong at the conceptual level, where it can also be corrected — by more sensory evidence.

This is why Objectivism treats the standard catalogue of illusions as confirming the senses rather than impeaching them. Each one is explained by identifying additional facts about the object, the medium and the sense organ, and every step of that explanation rests on further sensory evidence. An investigation that ends by concluding the senses are unreliable has sawn off the branch it was standing on.

The form of perception

The most common objection is that we perceive things only in a particular form, determined by the kind of sense organs we happen to have, and that this form stands between us and the object as it really is.

The Objectivist answer is that a form of awareness is not a barrier to awareness; it is what awareness consists of. To perceive at all is to perceive by some means, and a means has an identity. Consciousness has identity just as everything else does, and demanding awareness with no specific means is demanding awareness that is nothing in particular. What the senses give is the object, grasped in the form appropriate to the organ that grasped it. Colour is not a fiction imposed on a colourless world; it is the form in which a certain range of the object's properties registers on an eye of a certain kind.

The rejection of that answer is where the primacy of consciousness enters epistemology, and Rand regarded Kant as its systematic exponent. See Realism and Objectivity.

Validation, not proof

The senses cannot be proved valid, because proof is a process of deriving a conclusion from evidence already accepted, and all evidence traces back to the senses. Demanding a proof of the senses asks for a foundation beneath the foundation.

What can be done instead is validation: showing that the concept is axiomatic, that it is presupposed by every attempt to challenge it, and that the challenge is therefore self-refuting. The skeptic who says the senses might deceive us has to gesture at evidence of deception, and that evidence reached him through his senses. See Axioms, Proof, Skepticism and Fallacy of the stolen concept.

Common misunderstandings

That it means people cannot be fooled. They can, routinely. The claim concerns which faculty commits the error, and the answer is the one that judges, not the one that receives.
That instruments extending the senses somehow bypass them. A microscope, a voltmeter and a particle detector all terminate in something a person looks at. They extend the range of sensory evidence rather than replacing it.
That animals therefore have knowledge in the human sense. They have percepts. What they lack is the conceptual faculty, which is where knowledge, error and logic begin. See Consciousness and Man's nature.
That "the senses are valid" is a claim about the reliability of any given observation. A specific observation can be made carelessly, in poor conditions, or by a damaged organ. Identifying that requires the same faculty, used more carefully.

Discussion

References

Rand, Ayn. Introduction to Objectivist Epistemology, chapter 1 and the foreword. The Objectivist, 1967.
Rand, Ayn. "Kant Versus Sullivan", Philosophy: Who Needs It. Bobbs-Merrill, 1982.
Peikoff, Leonard. Objectivism: The Philosophy of Ayn Rand, chapter 2.
Kelley, David. The Evidence of the Senses. Louisiana State University Press, 1986 — a book-length defense of perceptual realism along these lines.

External links

Sensations at the Ayn Rand Lexicon

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