The first chapter suggested some reasons to favour Direct Realism over Indirect Realism and Idealism. Direct Realism has some common sense appeal. The other theories face troubling challenges, including the illusoriness of ideas. Chapter 2, however, showed that Direct Realism cannot rationally stand alongside the three premises of the Problem of Secondary Qualities: the Secondary Quality Spreading Principle (SP), the Secondary Quality Observation Claim (OC) and the Secondary Quality Non-Physicality Thesis (NPT). Something must give. This chapter considers SP, OC and NPT more carefully in hopes of pinpointing some weakness that can save Direct Realism.

Surprisingly, the most vulnerable premise in the Problem of Secondary Qualities turns out to be NPT, the supposed deliverance from science that physical objects do not possess secondary qualities. Some philosophers may cringe at the thought of challenging scientific dogma, but the present chapter explains the rationale behind this move. SP and OC hold up under scrutiny, and NPT isn't quite as scientific as you might expect.

The Secondary Quality Spreading Principle (SP)

The first premise, the Secondary Quality Spreading Principle, says that if there are immediate objects of perception that possess secondary qualities and these are not physical, then no immediate objects of perception are physical. One could easily mistake it for the argument's weak point. SP seems finicky, preoccupied with secondary qualities in particular, and unmotivated. So what if perceiving secondary qualities means perceiving ideas or sense-data? Aren't secondary qualities exceptional? Why extrapolate an analysis of secondary quality perceptions to perception generally?

SP holds its ground surprisingly well. Recall that SP entered the discussion as a special case of Howard Robinson's more general spreading principle. Robinson based his claim on causal and physiological continuities between veridical perceptions and slight illusions. Your visual perception of a clock's face, for example, seems pretty much the same whether you observe it in a bleary-eyed stupor or with the wakeful clarity of a late-morning cup of coffee. Similarly, George Berkeley bases a version of SP on the phenomenological continuity between secondary and other properties. Witness the following exchange between Berkeley's characters, Hylas and Philonous: