5.1.1.1 Skeptical argument against reality

To start with, what is the fundamental reasoning in support of Idealism? There are several justifications, but one of the simplest is this. Using the formal logical structure called “affirming the antecedent” (AKA - modus ponens), skeptics of claims about an external reality pose this argument:

• If we don't know that reality is not an illusion, then we don't know that external objects really exist
• We don't know that reality is not an illusion
• Therefore, we don't know that external objects really exist

Its formal structure is:

• If P, then Q
• P is true
• Therefore, Q is true

Although it seems sound, similarly formed arguments can resolve to a completely different conclusion (see Moore, later in this paper). In particular, the second premise (we don't know that reality is not an illusion) is the basis of the entire debate - it cannot be simply assumed.