A principle is a relationship, in every case.
When two (or more) items interact, the rules that describe their interaction can only be discovered, illustrated or applied by recourse to the behavior they cause in each other. A single item cannot teach a principle.
Hence, The Principle of Unity, taught to each of us before we can remember: Unity is Plural, and at minimum, Two. We know darkness, because of light. We know of coldness, because there is warmth. As our minds were forming, we only become aware of ourselves, when we recognized that there are others. Who do you suppose is the first person you can remember?
The Principle of Unity has been passed down through history in many ways, some accurate, some not: There is only One; Opposites come in pairs; 1 + 1 = 3; the Law of Identity. 
Many similar expressions exist, because the root notion is the foundation of all knowledge: You can’t even begin to recognize a principle, until at least two items interact accordingly. That’s the only way Principles manifest.
A rather cumbersome dialect holds it as the Principle of Category: pictures in the mind (vidéa) associate, based on their qualities, into different classes. These classes have sub-classes, based on more specific properties and qualia, each member of which predicates the qualities used to recognize the super class. Ugh! Latin. Is anyone bored, yet? Who can possibly trust the language of the nation that invented long-term inflation, some MM+ years ago?
As an example of a major weakness of this formulation: The qualities you select to define your categories are demonstrably arbitrary, a fact which went unnoticed for 1.5K years, and is still widely unfamiliar, 2K years later, and despite our continued use of their symbols, words, and models — or is it precisely because we still use them?
Thinking in terms of classes / objects / substances is why it can be so difficult to recognize that a dimension of our experience is missing from our language: reification.
A fatal, permanent error exists in the Latin formulation, as well: What’s the most-general class?
Answer 1: Oops, I can’t trust this logical formulation to provide useful results. Worse, I now know that I won’t be able to tell the difference between the useful and the useless, which is the only reason to have any logical formulation at all. (Back to the drawing board…)
Answer 2: Ainu / Allah / Brahman / God / Yahweh / Zen / The Rule(r) of Rule(s) / The Great Pumpkin
Translation: I neither know, nor care.Answer 3: Geometry (Pythagoras, to my knowledge, was the first to get this close – Kepler agreed)
Answer 4: A system of organization (Aristotle came up with this one – R. Buckminster Fuller agreed.)
Physical Principles 
Unlike our planet and moon, the principle that keeps them bound weighs absolutely nothing at all. Despite their ethereal nature, principles are completely real. Some have even said ‘reality’ is made of nothing else; there are no items, only principles. Modern Physics proves it’s possible, and the name of that idea is String Theory.
Though completely invisible, gravity operates on everything we know. As the tools of science extends our senses into previously hidden realms, we’ve learned that 99.99+% of reality is totally unavailable to unaided human sensory equipment.
It should be no surprise that Universe operates according to invisible recipes.
It seems that humans are designed to (re)discover them, however; such discovery is how humans survive, by employing some principles to improve and extend their biological functions, and being aware of others, as a way of avoiding mistakes. Some say that “man was made in God’s image” because our minds can develop ideas which match the “ideas” that God used to think the universe into existence. Perhaps more literally it means our minds can grasp, simulate and model the abstract relations we experience in the universe and which people really mean when they say “God.” Spinoza’s Pantheism, Deism and Einstein’s Non-Anthropomorphic Conception Of God are alternative names for approximately identical notions.
Speculation is unending about whether the Universe wants us to stay…and probably will be, as long as it hasn’t decided ‘no.’
Principles – the rules by which events occur – possess certain characteristics. Only a True Principle has all of them.
- Always operating everywhere
- Have no mass or energy
- Expressible mathematically
- Learning allows prediction
- Employing allows achievement
- Ignoring one can be fatal
- None interferes with any other
They can certainly oppose each other, as does a star, balancing gravity and radiation to exist. Throughout it (and, indeed, the Universe), both of the governing principles – matter attracts and radiation explodes – always operate … as does every other True Principle.
You can see this demonstrated in the operation of chemistry, as well: Electric forces between molecules operate according to highly predictable and eminently employable principles. All life is technology, designed in a manner to employ such principles, built of structures which resist deformations because of the strength and geometry of the specific principles which bind them.
Such structures are experienced as ‘substance.’ The geometry is their form. Their integrity is why we know they are real: We can ‘touch’ them, which means when we push on them they push back … though technically ‘touch’ never actually occurs; the resistance we use to notice something is real – the pushback – actually results from electrical interactions between items that aren’t in contact. 
When you sit upon a chair, it is ultimately electric attraction between the atoms in the chair which keeps it from collapsing. Your weight is transferred down, via electrical forces, into the chair or ground. This down-pressure converts to an out deformation – a bean bag seems like a structureless chair, but the tension in the fabric that keeps it together is ultimately what stops the down-into-out transformation, and thus stops your down translation. That is, the tension in the chair (the legs for a regular chair or the tensile strength of the bag for a beanbag chair) stop you from falling all the way to the ground when you sit down (and the chair keeps its structure.)
It is only the operation of structure – the resistance to deformation – which creates an experience of ‘stuff’ and simultaneously defines structure because of that experience. There aren’t really any things or substances. Only objects; event-complexes; something attended to.
Air, for example, doesn’t resist deformation at all. Though any one particle does have structure, the ‘substance’ of ‘air’ isn’t even there; it just moves out of the way, infinitely deformable.
This is the beginning of General Systems Theory; Principles + Motion + Shape + Experience = Reality.
