The term 'consciousness' is somewhat overloaded. I prefer 'subjectivity' to denote the strange fact that there is something there is like to be me--cf. Thomas Nagel's essay 'What is it like to be a bat?'. This subjectivity seems absent in the science of enlightenment thinkers who were driven to find the 'objective' laws of nature.
Nagel wrote a controversial book towards the end of his life called 'Mind and Cosmos: Why the Materialist Neo-Darwinian Conception of Nature is Almost Certainly False' where he argues that this very subjectivity cannot be fit into any current materialist theory. I tend to agree with this view--reducing subjectivity to a metaphenomena of complexity seems to be a completely hollow definition. Why does it exist?