Sentience is brought into being somehow from the evolution of sensing and acting; it involves being a living system with a point of view on the world around it. If we take that approach, though, a perplexity we run into immediately is the fact that those capacities are so widespread— they are found far outside the organisms that are usually thought to have experience of some kind. Even bacteria sense the world and act... A case can be made that responses to stimuli, and the controlled flow of chemicals across boundaries, are an elementary part of life itself. Unless we conclude that all living things have a modicum of subjective experience— a view I don’t regard as insane, but surely one that would need a lot of defense— there must be something about the way animals deal with the world that makes a crucial difference.
The senses can do their basic work, and actions can be produced, with all this happening “in silence” as far as the organism’s experience is concerned. Then, at some stage in evolution, extra capacities appear that do give rise to subjective experience: the sensory streams are brought together, an “internal model” of the world arises, and there’s a recognition of time and self.
What we’ve learned over the last thirty years or so is that there’s a particular style of processing— one that we use to deal especially with time, sequences, and novelty— that brings with it conscious awareness, while a lot of other quite complex activities do not.
Baars suggested that we are conscious of the information that has been brought into a centralized “workspace” in the brain. Dehaene adopted and developed this view. A related family of theories claim that we are conscious of whatever information is being fed into working memory,These views don’t hold that the lights went on in a sudden flash, but they do hold that the “waking up” came late in the history of life and was due to features that are clearly seen only in animals like us.
...certainly there’s an alternative to consider. I’ll call this the transformation view. It holds that a form of subjective experience preceded late-arising things like working memory, workspaces, the integration of the senses, and so on. These complexities, when they came along, transformed what it feels like to be an animal. Experience has been reshaped by these features, but it was not brought into being by them. The best argument I can offer for this alternative view is based on the role in our lives of what seem like old forms of subjective experience that appear as intrusions into more organized and complex mental processes. Consider the intrusion of sudden pain, or of what the physiologist Derek Denton calls the primordial emotions— feelings which register important bodily states and deficiencies, such as thirst or the feeling of not having enough air. As Denton says, these feelings have an “imperious” role when they are present: they press themselves into experience and can’t easily be ignored. Do you think that those things (pain, shortness of breath, etc.) only feel like something because of sophisticated cognitive processing in mammals that has arisen late in evolution? I doubt it. Instead, it seems plausible that an animal might feel pain or thirst without having an “inner model” of the world, or sophisticated forms of memory.
Subjective experience does not arise from the mere running of the system, but from the modulation of its state, from registering things that matter. These need not be external events; they might arise internally. But they are tracked because they matter and require a response. Sentience has some point to it. It’s not just a bathing in living activity.
By the Cambrian, the vertebrates were already on their own path (or their own collection of paths), while arthropods and mollusks were on others. Suppose it’s right that crabs, octopuses, and cats all have subjective experience of some kind. Then there were at least three separate origins for this trait, and perhaps many more than three. Later, as the machinery described by Dehaene, Baars, Milner, and Goodale comes on line, an integrated perspective on the world arises and a more definite sense of self. We then reach something closer to consciousness. I don’t see that as a single definite step. Instead, I see “consciousness” as a mixed-up and overused but useful term for forms of subjective experience that are unified and coherent in various ways. Here, too, it is likely that experience of this kind arose several times on different evolutionary paths: from white noise, through old and simple forms of experience, to consciousness.