Consciousness is identical to the presence of qualia, that there is something rather than nothing, that the lights are on.

‘Reductive’ means the assumption that all phenomena reduce to what represent them in an external model or linguistic description.

Science, language, and reason are fundamentally reductive. So in practice the reductive assumption is just denying that things have an existence which is not identical to our models of them. And since these are the respected modes of thought most people are unaware that they are committing to a non-trivial reductive assumption when reasoning about consciousness.