There are arguments confirming and denying that animals have consciousness. Some arguments say that not even all humans have consciousness, e.g. small babies.
Which observable signs do exist that indicate that another being (animal, baby, ...) may be conscious?
Following these signs, what would be the consequences of animals or some humans (not) being conscious?
If you mean consciousnesses as the existence of subjective experience ("Qualia"), then there's probably no way to distinguish between an entity with qualia, and an entity with out one. Even when we consider adult, healthy people, we can't know "if there's somebody in there".
The term p-zombie is used to describe (hypothetical) entities that don't have qualia, but appear just like regular people.
The only thing I can think of that such p-zombies can't do, is understand the concept of qualia. so in a society consisting of only p-zombies, I would expect this term wouldn't exist.
Only a partial list, but here are a few traits that lead people to believe X is conscious. Note that none of these are "necessary" traits, they're certainly not "sufficient", and all of these traits lie on a continuum (they can't be reduced to checkboxes). Also note that these are traits that lead people to believe X is conscious, not traits that make X conscious. While the two are surely correlated, this is an important distinction.
Learning: Does X show evidence of learning? If X is put in the same situation many times, does it do better on some criterion over time? Or does it make the mistakes? This criterion has been rolled in with adaptation to the environment.
Perception: Does X take its environment as input, in order to make decisions? It doesn't necessarily have to have eyes and hands, but does it react to its environment in a suitable way?
Intelligence: Is X able to solve 'hard' problems? Can it do math or play chess? I don't really like this criterion because it is quite subjective, yet it seems humans are willing to base their decision on subjective criteria like this. Imagine what a human living 2,000 years ago would think of today's computers-- would they be deemed conscious?
Planning: While humans and animals make plenty of spur-of-the-moment decisions, they also seem to make quite a lot of decisions that involve foresight. Does X show foresight, or does it just hill climb?
Creativity (insight): Can X abstract its own knowledge, such as making analogies? Can it apply solutions from one problem to another similar-looking problem? Can it form novel solutions based on abstract knowledge, or can it only solve novel problems by chance and repetition?
Similarity: We "know" that humans are conscious. The more similar something is to us, the more likely we are to judge it as conscious. We are quite adept at distinguishing biological movement from aritificial movement, and the same goes for language. This criterion is a bit different from the others on the list, because we often believe the other traits to defining features of consciousness, where this is merely an indicator.
What would be the consequences of some animals or humans not being conscious? Probably not much, but that's my opinion. The world goes on as it has, now with new labels. Our conceptions of ethics and what it means to be human would change, but the outside world remains the same.
The fast answer is, "the being's ability to recognize itself in a mirror."
There are a handful of animals that can do this (Elephants, Dolphins, and some primates). The most common way to test whether an animal apprehends that its reflection is an image of itself, is to mark the animal and observe whether it attempts to remove the mark when it sees the mark on itself in the mirror.
Consciousness
Consciousness has degree of sensitiveness with abilities to:
Aware of something
Feeling something
Degree of sensitiveness
Different degree of perceive differences can be measured by:
Answering "how many dimensional can be perceived?" The more surfaces with different dimensional can be perceived by something, the more something can be considered having more consciousness (more aware)
Answering "how many for the details of specific surface of specific dimensional can be perceived?" The more the details of specific surface of specific dimensional can be perceived by something, the more something can be considered having more consciousness (more aware)
Different degree of feeling (focusing on) something can be measured by:
Answering "how many dimensional can be focused on?" The more surfaces with different dimensional can be focused on, the more something can be considered having more consciousness (specific feeling with more intense).
Answering "how close for something can be focused on?" The closer for the surfaces with different dimensional can be focused on, the more something can be considered having more consciousness (specific feeling with more intense).
Any kind of justification to make sure whether a being has consciousness must be closer to these requirements.
Comparison
Animals and plants having abilities to differentiate for the purposes to fulfill their needs.
Animals and plants having abilities to focus on specific directions for the purposes to fulfill their needs.
- Therefore animals and plants are having consciousness, but with different degree of consciousness.
If we make a comparison to human. Human having abilities to do as animals and plants did but with wider ranges. It puts human as being with higher consciousness.
Compared to zombies, since zombies have abilities as already described, then zombie must be considered having new consciousness differently to previous consciousness (whether it might be considered from subconscious or being possessed).
Intelligent
Switching off consciousness
We could experience it when we were sleeping, meditating or being hypnotized.
If other beings can act intelligently also what they perceive and what they focus on something can be switched off and switched on again, this indicates strongly that those beings have consciousness.
There is degree of consciousness.
According to First Justification, human, animals & plants are conscious being with different degree of consciousness
Whether a device (body) is possessed by subconscious or conscious, it may be asserted as conscious being.
To be more specific, according to Final Justification, human, animals and any other beings that may be switched off for their consciousness using general anaesthetic, they can be considered as conscious being.
A slightly different definition from what other answerers use of "conscious" is "able to feel pain." Varner gives six suggestive criteria:
If you follow that link, you can see that there is strong evidence to believe that at least vertebrates feel pain. The consequences are (in Varner's opinion) a moral imperative for veganism.
Maybe we are making a mistake by saying that consciousness is something which fully exists or doesn't at all, like a on/off thing.
Maybe even a bacteria has some degree of consciousness, given they do feel things and they do have a sense of self. Maybe even an atomic particle does. Maybe consciousness is a property of the universe, maybe it's the view of things from the inside, while physical science is the view from the "outside".
Of course this is a problem if we want to respect all sentient beings! We already aren't because our immune system is killing thousands of bacteria right now :O
(I'm sorry but I can't still post in the comments)
