 Visualizing first. It is not particularly hard to see why visualizing satisfies condition (a). Typically: one can decide to visualize (or stop visualizing) something; the process is not driven by perceptual stimuli; and at least some parts of the visualization process are conscious. There might be cases in which visualizing is not under voluntary control, is stimulus-driven and, maybe, even entirely unconscious. This, however, is not a problem, since we know that there are clear cases satisfying (a).