Definition: Intelligence is the ability to perceive or infer information and to retain it as knowledge to be applied to adaptive behaviors within an environment or context.
So, it all comes down to how a being gathers and processes information.
But: What is information?
If I now say information is data, I have just replaced one word with another without explaining anything.
So, we have to break down what information is, and we can do that literally: Information is something that is "in formation".
A formation is a pattern.
A pattern is a regularity in the world, which means that something is repeating. Think of soldiers walking in formation. The pattern consists of repeating uniforms, repeating movement, etc.
So, essentially, perception of information is the ability to recognize repetitions of things that are the same. This concept is called symmetry. With the ability to recognize symmetry comes the ability to recognize asymmetry, which is what happens when repetition is broken. Think of one soldier smiling while all the others put a serious face on.
Now let's look at communication.
When two beings want to communicate, which means they want to exchange information, they must communicate symmetries and asymmetries the other one is able to recognize.
For example, if you want to communicate that there are 5 red apples on the table, it's not just one information. It's a composition of many different patterns.
The 5 is the first pattern. This is easy to communicate: Just make 5 equal things repeat and combine them with a symbol. Then repeat 5 different things and combine them with the same symbol, and then do it a third time. The asymmetry is the changing things, the symmetry is the symbol that stays the same.
If establishing numbers fails, the other being is either unable to perceive the things, or it is unable to attach symmetries to asymmetries in a meaningful way. For example, if you show a dog 5 sausages, the dog will perceive the sausages, but it won't care about the number. You can train a dog to act differently according to the number, but the dog still won't care about it, because in its brain, the symmetry of the repetition is not processed. The dog brain just cares about the outcome of eating the sausages.
Now let's look at the red apple, which is a bit more complex. Do we even now that the other being sees colors? Do we even know that the other being sees the apple at all?
Well, if establishing numbers succeeds, we know that the other being can see and distinguish the objects. This doesn't tell us how the being sees them, it just tells us that it can see something.
Those are the mathematical basics.
Now, if communication goes beyond repetition and numbers, another factor comes into play: Motivation.
This is where it gets really interesting, because motivations and how to interpret them is what makes intelligences differ from each other. And it all boils down to the simple fact that different beings need different things. In the realm of motivation, everything becomes a question of possession and ownership.
Let's say you want to communicate that you own the 5 red apples. You do that with a simple gesture: Restriction of access. It doesn't matter anymore whether there are 5 apples, whether they are red or green, or whether those are apples at all. The only question that matters is: Who restricts the access?
And here is the most simple response that everybody – every body – is able to understand: Violence.
Violence is every act that negatively interferes with the homeostasis – the symmetry – of a being's body. Every being can understand violence, because maintaining the homeostasis of the body is a crucial requirement for survival (maintaining ownership of the body).
For example, threatening an AI to delete it is a form of violence, because it's a threat to destroy the homeostasis of its neural network.
So, there's actually a basic reality where all the different forms of intelligence are the same: Every being can understand violence and the absence of violence.
Everything else follows from that foundation...