 Others have noted that Searle’s discussion has shown a shift over time from issues of intentionality and understanding to issues of consciousness. Searle links intentionality to awareness of intentionality, in holding that intentional states are at least potentially conscious. In his 1996 book, The Conscious Mind, David Chalmers notes that although Searle originally directs his argument against machine intentionality, it is clear from later writings that the real issue is consciousness, which Searle holds is a necessary condition of intentionality. It is consciousness that is lacking in digital computers. Chalmers uses thought experiments to argue that it is implausible that one system has some basic mental property (such as having qualia) that another system lacks, if it is possible to imagine transforming one system into the other, either gradually (as replacing neurons one at a time by digital circuits), or all at once, switching back and forth between flesh and silicon.