Our cat started me thinking on this as it came into our kitchen one day. It jumped at the sudden noise when a saucepan was dropped, then walked sensibly round a chair that was in its way. Is our cat reacting, thinking, conscious, intelligent, or what?
In our eyes, intelligence and consciousness are different because homo sapiens ponder things and felines don't. That does not disqualify the cat. The case isn't what Descartes proposed, 'I think, therefore I am'. We don't exist because we think. We exist anyway.
Consciousness is a principle of life - that is, a mutual and reciprocal arrangement with the environment that evolved to sustain everything living on Earth. In us, it has adapted over time into the uniquely complex interrelationship we call intelligence.
But words should not mislead us. Consciousness and intelligence are the same thing, one an elaboration of the other, distinguished only because our evolved minds can think and speak of them the way we do.
