 David Chalmers (1996) famously argued that any possible brain-process account of consciousness will leave open an “explanatory gap” between the brain process and properties of the conscious  experience.[10]  This is because no brain-process theory can answer the “hard” question: Why should that particular brain process give rise to that particular conscious experience? We can always imagine (“conceive of”) a universe populated by creatures having those brain processes but completely lacking conscious experience. A theory of consciousness requires an explanation of how and why some brain process causes a conscious experience, replete with all the features we experience. The fact that the hard question remains unanswered shows that we will probably never get a complete explanation of consciousness at the level of neural mechanism. Paul and Patricia Churchland (1997) offered the following diagnosis and reply. Chalmers offers a conceptual argument, based on our ability to imagine creatures possessing active brains like ours but wholly lacking in conscious experiences. But the more one learns about how the brain produces conscious experience—and such a literature has emerged (for some early work, see Gazzaniga 1995)—the harder it becomes to imagine a universe consisting of creatures with brain processes like ours but lacking consciousness. This is not just bare assertion. The Churchlands appeal to some neurobiological detail. For example, Paul Churchland (1995) develops a neuroscientific account of consciousness based on recurrent connections between thalamic nuclei, particularly between “diffusely projecting” nuclei like the intralaminar nuclei and the  cortex.[11]  Churchland argues that thalamocortical recurrency accounts for the selective features of consciousness, for the effects of short-term memory on conscious experience, for vivid dreaming during REM (rapid-eye movement) sleep, and other “core” features of conscious experience. In other words, the Churchlands are claiming that when one learns about activity patterns in these recurrent circuits, one can no longer “imagine” or “conceive of” this activity occurring without these core features of conscious experience occurring. (Other than just mouthing the expression, “I am now imagining activity in these circuits without selective attention/the effects of short-term memory/vivid dreaming/…”).