tratum of extension. Be the sensible quality what it will--figure, or sound, or color; it seems alike impossible it should subsist in that which doth not perceive it (Berkeley 266).
In Berkeley's view, anything that the human mind can imagine (unaided human flight, a Cubs league pennant) has the same reality as that which can be inferred by analogy or confirmed by experience (driving a car, a blue goat). But this is important: That sameness of reality = that it has no independent reality whatever. Instead, it is an object of imagination conceived (perhaps inevitably, given that the evidence of material reality is so compelling to immediate experience, but nonetheless wrongly) as perception. One says that one hears the coach: wrong (Berkeley 271). One hears a sound and by mental organization of analogy and association comes to the conclusion that the sounds are consistent with one's experience of horses and coaches. The reality of the thing-in-itself, or what makes the thing real, is perceived by the senses or conceived and abstracted by the mind. But that is different, in Berkeley's view, from having a self-sufficient reality. Reality is always and everywhere and only tied to sense experience, which resides in the human mind.
In the dialogue, Philonous traps Hylas into admitting that he cannot explain how ideas are produced by recourse to brain impressions, since they themselves are a product of mind work, i.e., of the human sensibility at work. To "talk of ideas imprinted in an idea cuasing the same idea [] is absurd" (Berkeley 274). The ability to form ideas about objects, even the mind or brain itself, is not the same as the ability of the objects to exist independently of the mind.
Then, a surprise. Philonous abruptly (seemingly) reverses himself and urges Hylas to look about him at the abundance of evidence for material reality in particular with respect to the cosmic order of nature. Philonous makes much of delightful ver...