It seems like quite a few of you are writing science fiction (or even just take your fantasy seriously), and might want to know this.
Note: If you don't care about accurate usage of words and scientific terms in your writing, to each her/his own, and you can probably skip this. But if you want to get things technically right, please read on...
There are two meanings of the word "sentient", one being the actual meaning, and one being an unfortunately common misuse of the word.
Sentient just means capable of sensation and perception of the world around.
Worms and animals like that might be a gray area, but every higher being is sentient (even really dumb ones like flies and politicians).
Things below worms like fungi and plants are non-sentient (that doesn't mean they don't have complex reflexive responses, but they do not comprehend and process the information in a way that creates the mental state of perception, which is inherently linked to at least very rudimentary intelligence). See later in the post for more details on lower and higher states.
Sapient is usually what people really mean when they mistakenly say "sentient". Sapient means "wise", or possessing a roughly human level of intelligence and insight into existence and the world around them.
Thus our phylogenic name: Homo sapiens [+sapiens subspecies, to differentiate from Neanderthal if you like]
The usage of sentient vs. sapient to describe an alien species is almost as good a litmus test as you can get to find out if a science fiction writer did her or his homework.
But wait, there's more!
If you want an even more subtle and nuanced way to understand or reference different matters of thought and cognition, philosopher Daniel Dennett came up with a naming scheme to delineate them. Here's a fancy chart somebody made (pdf), but I'll summarize them here:
Darwinian creatures:
Behavior developed only through evolution, using natural selection to guide functional changes. A species can adapt and change its behavior over generations by failure of bad behavior to lead to reproduction, but individuals can not modify their behavior through learning (only respond reflexively). Note: Individuals may be sensitized to stimuli, and complex interactions can occur, but while these behaviors may resemble intelligence, they are not true intelligence (no real learning is taking place, just programmed response to variables).
- This class of creatures includes fungi, plants, protozoa, bacteria, and some lower forms of animals like perhaps sponges, jellyfish, and some kinds of worms.
- These creatures are inherently non-sentient, and non-intelligent (lacking the most fundamental building block- learning)
- Through evolution, a creature may evolve either out of or into this state (like possibly oysters, because they are non-motile, they have lost their brains and now behave reflexively, showing no signs of intelligence). It's not a one-way street.
Skinnerian creatures:
Behavior developed through "trial and error" based learning (the most primitive form). They have basic sensation, and at least the most rudimentary intelligence (some kind of brain to process information and adapt, as a computer neural network can). These beings can feel pleasure and pain, and randomly act upon the environment to receive stimuli, then reinforce those behaviors that yield more reward and avoid pain, and iteratively adjust behavior.
- This class of creatures includes many worms and very simple animals (many insects), as well as true Synthetic Intelligence (using adaptive neural networks to change behavior and learn).
- These creatures are inherently sentient, because experiencing pain is a prerequisite to this kind of learning, and have at least basic intelligence. Although they are not necessarily sentient in the misused sense meaning sapient.
- It's hard to determine for sure where this class of creatures ends, because there's a fuzzy line between this class and the next one. Where it begins can be demonstrated based on conditioning.
Popperian creatures:
Behavior is developed not just by trial and error, but by modeling the world around them internally (basic imagination). A great quote that explains the difference is that these creatures can:
"allow hypotheses to die in their stead"
They don't have to try everything out to figure out that it's probably a bad idea. This is comparatively very advanced intelligence, and is only certainly present in higher animals.
- This class of creatures includes most Birds, Mammals, Reptiles, Amphibians, Fish, some Mollusks (like Octopus), and other conventionally regarded 'intelligent' animals.
- This ability usually only exists in larger animals that can afford the comparatively larger brains required to model the world around and problem solve. However, smaller animals can surprise us, like the Portia spider, which has evolved very high intelligence and advanced problem solving by necessity because it hunts other spiders and has to outwit them by performing advanced pathfinding. There are clearly computational limits, however, as this Spider can take its sweet time to process its path.
- There's also no clear line between this class and the next... these are more like fuzzy guidelines.
These are creatures that think about thinking. They're introspective, and they can not only model environments, but change the way in which they think about and model environments, modeling their modeling methods.
It's very meta, and typically very human. There's good reason to believe that this isn't a simple product of biology alone, but of biology (providing the raw processing hardware) in conjunction with linguistic systems that create easily comprehended and compressed archetypes and symbols from ideas (the software). This is where neurolinguistics, and advanced concepts like that come in, and some really great science fiction has been written on the subjects.
- Creatures accused of possessing these abilities are humans (well, most of them), cetaceans (whales, dolphins, etc.), and perhaps mice if you're a fan of Douglas Adams.
- Because this ability is based on language, it can be upgraded or acquired through training in language (as has been established with some of our fellow great apes, from Gorillas to Bonobos and Chimps), and it can also fail to develop when children are deprived of language (don't try this at home).
- It really begs the question of whether Sapience is something you just have, or something you learn to realize a potential. Higher levels are also speculated, but I'll stop here.