We suggest that consciousness (C) should be addressed as a multilevel concept. We can provisionally identify at least three, rather than two, levels: Gray's system should relate at least to the lowest of these three levels. Although it is unlikely to be possible to develop a behavioural test for C, it is possible to speculate as to the evolutionary advantages offered by C and how C evolved through succeeding levels. Disturbances in the relationships between the levels of C could underlie mental illness, especially schizophrenia.