The difference between conscious and unconscious processes is a hard issue. Usually in the literature it is argued that without a mask, participants are conscious of the stimulus, but with a mask they are not. However, how we know that participants are not still conscious of the stimulus after it is masked? In fact, all we know with real reliability is that they can no longer report on what the stimulus was. Here we argue that without a mask, subjects can report what the stimulus was. With a mask, they cannot. This captures the crucial difference and it does so in a way that is unproblematic in itself and neutral with respect to the bigger underlying issues. The book includes a series of experiments and simulations on this subject. The model is new and uses neuromodulation and dynamic computation. It matches the findings in the book and previous experimental studies, with new and/or better results compared to previous models in this area (which are fairly rare).