THE ARGUMENT FROM ILLUSION (and Aftermath)

Background: We tend to unreflectively believe in an external, material world that existes independent of us or our perceptions.  But do we have good reasons for believing in such a “mind-independent, material reality”?

A first stab (DIRECT REALISM): Surely our beliefs in mind-independent material things are justified.  After all, we see them (or hear them, feel them, smell them, etc.)

The argument from illusion is meant to shake our faith in this common-sense, “naively” realistic point of view.  Its aim is to show that we don’t directly perceive external, material things after all, but rather mind-dependent entities called “sense impressions” or “sense data.”

The Argument:

(1)  S’pose you are hallucinating a pink rat.

(2)  Then you must be seeing something.

(3)  But what you see corresponds to no external material object.

(4)  Rather, it must be an internal, immaterial object. (a “sense datum” or impression)

(5)  But your experience is the same as it would be, if you were really looking at a pink rat.

(6)  So what it is that you see is the same in each case.

(7)  So what it is when you are really looking at something (e.g., a pink rat), all you ever really see are immaterial sense data or impressions.

                                                                                                            Q.E.D.

The Aftermath:

(8)  It follows that you must infer the existence of an external, material world from your inner sense data.

IDEALISM (Berkeley):

(9)  One cannot justifiably infer the existence of an external, material world from sense data.

            (All the properties we normally attribute to an external world are really just the properties of sense data.)

 (10)  So there is no justification for believing in an external, material world.

[Idealism is a form of skepticism.]

PHENOMENALISM [A “skeptical solution” to the problem of an external world.]

(11)  Instead, all of our statements about things in an external world should be reconceived as statements about bundles of sense-data.  That is, what we speak of as material objects are really just logical constructions of sense data.

[Indirect] REALISM (Bertrand Russell): You are entitled to believe in the existence of an external, material world after all, because of the durability, persistence, and inter-subjective character of sense-data.  External material objects are the [unseen!?] causes of our sense data or impressions.