So somehow, a physical system - the brain, creates or is correlated with those conscious sensations. Explaining this in terms of physics, function, computation, math, etc is hard because those fields lack any such sensations. An equation doesn't feel heat or see red, nor do atoms bumping around in the void. So from whence do those conscious sensations come from? Do they spontaneously emerge somehow from the complexity of a nervous system? Is the right sort of flow of electricity result in the smell of a rose? Is it the right algorithm that implements consciousness, if you just arrange the bits just right?