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.

• 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.

Word meanings do change over time. It's not really helpful to cling to an older meaning of a word as being correct.

It depends on the situation, but I'm not just appealing to original meaning here, rather the technical meaning (which is quite a bit different). "Sentience" used to mean sapience is more informal, and unprofessional. It's as bad a habit in science fiction as rubber forehead aliens.

If you're a strict descriptivist, you might as well say "their", "they're", and "there" are all the same word, because most people don't seem to know how to use them properly. And yet, few writers take those claims seriously. I don't know if you fall in that camp, but some things are just wrong and don't benefit the writing itself (particularly within circles that know better).

There is a pretty wide gap between science and science fiction, and I think it's useful for everybody to narrow that gap by minimizing mistakes where they're easy to correct with a bit of research. It makes the writing better, informs and inspires the public better, and benefits science as well.

Some people write whatever they feel, and don't care about accuracy or doing research, and I get that. If that's your thing, that's cool. I'm just trying to give some pointers to people who want to be more rigorous.

As to what is helpful:

What is helpful is what clears up ambiguity and improves understanding of the practical nuances involved.

A word being represented with two similar meanings, which are also confused with consciousness, and any number of other terms just complicates communication.

The more useful meaning of sentience is the exclusive one, which doesn't have a readily available alternative.

When we talk about being capable of sense perception, the only appropriate word really is sentient.

So by taking that word and using it for a concept that already has plenty of other terms (most prominently, sapient), you weaken the ability of the author to communicate that original concept to the reader, because now any attempt to appeal to it results in ambiguity and confusion.

I suspect that the mistake in using the word "sentient" may not be confusion with "sapient", but rather confusion with "self-sentient": being self-aware, as described under "Gregorian Creatures" in the first post above. That, at least, has been my impression of how the term has been used.

Ah, yes, you may be right. I could see it evolving from that. Although I'm not sure if I've ever seen "self-sentient" used, but it may have been used in much older genre work that I'm not familiar with.

Somewhere along the way the "self" part got lost and it got muddled up with the other meaning - very plausible.

I suspected it came from the (medieval?) misconception that only humans have intelligence (or a soul), and that other species were automatic and incapable of sensation, like machines.

Although I don't know where this itself came from, since the understanding of at least three distinctions in categories of soul goes back at least to Aristotle (almost 400 BCE), with plants possessing the capacity for nourishment, lower animals possessing in addition to that sentience and varying intelligence and imagination, and humans possessing in addition to that rationality.

It's commonly believed that much of human knowledge was lost during the dark ages after the fall of the Roman empire, but I'm not so sure that's really the case. I don't think scholarship of Aristotle ever completely fell out of practice, and we have surviving manuscripts of De Anima from the middle ages.

Puzzling.

Wait, wait, WAIT! .... octopodes are mollusks!?

Thanks. This is very helpful to me, actually. (The Sapient thing, not the mollusk thing... well, that's interesting too.)

Apart from making sure your writing is correct, learning the true meanings (and root-words) of words, and learning new words, is nice to learn purely as sources of creativity when thinking of ideas for stories or game mechanics.

Thanks for posting - I found it interesting.

Wait, wait, WAIT! .... octopodes are mollusks!?