Some years ago I gave a talk called “Consciousness and the Transhuman”, in which I discussed important connections between questions of neuroscience, advancing cognitive technologies, human augmentation and animal rights. Around the same time, philosopher David Pearce was beginning to popularize the concept of the “Hedonistic Imperative”, being a drive to eliminate animal and human suffering through technology. Now I would like to move the conversation forward with the introduction of a new conceptual tool: The Continuum of Sentience.

First, let us begin with a point of reference, which is to say the nature and significance of sentience. For our purposes here, let’s define sentience as (the capacity for) subjective perception and experience. Ethics are irrelevant in the absence of sentience, as all known ethical systems are based upon the avoidance of unnecessary or unjustified suffering, and only sentient beings are capable of suffering. For example, it is impossible to act unethically toward an inanimate object per se, although it may be possible for one’s actions toward an inanimate object (e.g. stealing or destroying it) to be unethical if they may cause suffering to sentient beings. Therefore our point of reference is zero sentience, which is also the point at which ethical rules do not apply.

From that point on, things get complicated. There are arguably different degrees and types of sentience, suggesting different degrees and types of ethical implication, and we must understand them if we wish to act ethically in a world of rapidly developing AI technologies (or indeed to act ethically toward any living thing). The Continuum of Sentience (CoS) is a single, broad measure of multiple correlated phenomena, being physiological and behavioural complexity, subjective experience, capacity for suffering, degrees of consciousness, and arguably life itself. Following the principles of good science, we should not rely solely on one type of observation when assessing an entity’s degree or type of sentience. Our understanding of the relationship between cognitive abilities and physiological structures or system architectures may be incomplete, and any entity’s reports of their subjective experience may be misleading. By observing and correlating multiple measures of (1) physiological similarity to known cognitive architectures, (2) behaviour, and (3) subjective report, we can develop an increasingly reliable overall measure of sentience.

But what are these “degrees and types of sentience”? How can sentience be anything other than a unitary phenomenon, simply existing or not? “Degree” is a question of the characteristics associated with a particular sentient process. Philosophers such as Daniel Dennett and Thomas Nagel have long noted that conscious awareness has content, which is to say that in order to be aware you must be aware of something. We may therefore consider a perceptual process representing richer content (e.g. high-resolution colour images and audio, versus low-resolution grayscale with no audio) to be “more sentient” than a less rich one, although perhaps the proper or more accurate terminology would be “content-rich sentience”. This essential level of sentience, no matter how content-rich, does not necessarily require reflexive consciousness, awareness of one’s own mental contents, sapience or capacity for explicit logical reasoning.

Such “higher forms” of sentience are the different types referred to earlier. The most advanced forms of intelligence that we currently know are capable of complex reasoning and linguistic ability, and such capabilities go hand-in-hand with historical terms such as “sapience” and “consciousness”. Unfortunately such terms are operationally ill-defined (a simple fact which has given rise to entire literatures of debate), and so for the purposes of the CoS we will refer only to higher sentience types (HST), defined by specific characteristics, capacities and mechanisms. The most fundamental HST mechanism is recursive processing, also known as metarepresentation and Higher Order Thought (HOT) in the psychological literature. The idea is that some systems are capable of representing some part of their own inner workings, and that such metaknowledge is the basis of self-awareness. Humans have a tendency to imagine that their metaknowledge is more or less total, when it most emphatically is not, and much of our own neurological (and arguably cognitive) activity is opaque to us.

To summarize, the Continuum of Sentience ranges from entities with the barest glimmerings of perceptual awareness on the one hand, to beings capable of rich phenomenological content, self-awareness, deep access to their own cognitive resources, complex linguistic and reasoning powers on the other. Furthermore, the Continuum also acts as a measure of ethical responsibility for all who would seek to avoid causing suffering to others. Of course one may decouple ethics from suffering and claim that it may be ethical to cause suffering to highly intelligent and aware organisms, but such a position is rarely held (or at least made explicit) in the world today. Arguments that certain levels or types of suffering may be justified under particular circumstances are tangential to my purpose here today, which is simply to introduce a conceptual tool for considering the cognitive capacities and associated rights of any intelligent system or organism.