How Magic Works: The Law of Similarity

Q: Is the walnut really medicine for the brain?

No — and that's the honest part of the story. The old doctrine of signatures, which read a plant's shape as a sign of its use, was a way of reading the world, not a proven pharmacy; the resemblances were usually noticed after a use was already known. We give it as a window into how the law was understood, not as medical advice.

October 21, 2026

By Hank, Graduate Gemologist & Professional Spell Caster | Crystal Conjure Magic

How Magic Works · Part 3 of 7 — the real principles behind every spell, one law at a time.

Here is the whole of it in a single line: things that resemble each other are magically connected. A likeness is never only a likeness — it holds a real thread to whatever it looks like. Magicians have trusted this for as long as there has been magic, and almost every spell you will ever meet leans on it. It has a plain old name, the Law of Similarity¹, and once you have it, you understand why spells are so full of colors and shapes and images. Like belongs with like.

That one idea shows up in three ways. Together they are most of how a spell is actually built.

First — a thing’s shape or color tells you what it is for. This is the oldest herb-lore in the world. The walnut, stripped of its husk and shell, is the very image of the brain — two folded halves, a dividing crease — and so it was held to have power for the head.² Eyebright, its small flower flecked like a bloodshot eye, was the herb for the eyes; a heart-shaped leaf was for the heart. The old practitioners read the whole green world this way, as a book in which every plant wore its purpose on its skin. Color works the same way: we reach for red when we mean the heat of love, green when we mean the life of growing things, gold when we mean the wealth of the sun — the shade that already resembles the wish. The resemblance is a sign: this thing, which looks like that thing, has a share of its power.

Second — a likeness can stand in for the very thing it resembles. Make a small doll in a person’s likeness and that doll becomes a stand-in for them: tend it gently, warm it, send it comfort, and that care travels to the one it was shaped to resemble. This is the poppet — the true folk doll, which is mostly an instrument of healing and protection, whatever the movies made of it. The same move lets a caster work with a figure pressed in wax, or an image set on the altar, to reach a person far away. The likeness is a handle; take hold of it, and you take hold of what it pictures.

Third — to act a thing out is to help bring it about. Burn a cord in two to cut a tie that binds you; let an obstacle shaped in wax melt down to nothing in the flame; perform, in small, the ending you are asking for. In old rain rites people would make a little storm of their own — a drum for the thunder, a handful of water flung into the air for the rain — and ask the sky to answer in kind. You enact the outcome in miniature, and the world is invited to follow.

Three ways, one law. And this is exactly why a spell asks for a particular color, a particular shape, an image of the thing hoped for. None of it is decoration or stage-dressing. The red of the candle resembles the passion it’s for; the shaped figure stands in for the person; the small acted-out ending imitates the real one you want. The caster is doing, on purpose and with care, the single oldest thing magic knows how to do: building a careful likeness, so that like may call to like.

It is worth remembering how deep this runs. It is older than writing — the first images humans ever made, painted deep in caves, were likenesses of the animals they lived by. And you have never truly unlearned it: watch a child with a doll, feeding it and mending its hurts, and you are watching someone who has never once doubted that the likeness holds a little of the life of the thing. The practitioner simply keeps that knowledge, and uses it well.

This is the first of the two ways a spell travels the connected world — by likeness. The second is by contact — reaching a person through something that was once truly a part of them, a name, a lock of hair — and that is the next law in this series.

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The Three Laws So Far

We’re halfway through the foundations, so here’s the whole thread in one place — the three laws we’ve met, and how they fit together.

1. The Law of Correspondence — the map. The world is one connected whole, and things that share a nature belong to the same family: the sun, gold, the lion, and the human heart are all one kind of thing wearing different faces. Correspondence is the map of those kinships — what belongs to what. As above, so below.

2. The Law of Sympathy — the reach. Because the world is so connected, a spell can act on a thing by acting on its kin — and distance is no barrier. This is how a casting reaches a person in another town or another country: not by crossing the miles, but by taking hold of something joined to them. Sympathy is how a spell travels that map.

3. The Law of Similarity — the likeness. The first of the two ways a spell travels the map: through resemblance. Things that look alike are connected, so a likeness — a color, a shape, an image — can reveal what a thing is for, stand in for it, or help call it into being. Like belongs with like.

Put simply: Correspondence tells you what is connected, Sympathy lets you reach it, and Similarity is the first way you reach — by likeness. Next in the series is the second way: the Law of Contagion, which reaches a person through something that was once a part of them.

What to Remember

The Law of Similarity: things that resemble each other are magically connected. Like belongs with like.
It shows up three ways: (1) a thing’s shape or color reveals what it’s for (the walnut for the brain, red for passion); (2) a likeness can stand in for what it resembles (the poppet); (3) acting out a thing in small helps bring it about (the cut cord, the small made rain).
It’s why spells use color, shape, and image — each is a likeness, chosen on purpose, not decoration.
It’s the oldest magic there is — read in every herbal, painted in the first caves, and known by every child with a doll.
It’s Sympathy’s first way — by likeness. Reaching by contact (a name, a possession) is the next law, Contagion.

Frequently Asked Questions

What is the Law of Similarity?

The principle that things which resemble each other are magically connected — like belongs with like. It works three ways: a thing’s shape or color shows what it has power over; a likeness can stand in for what it resembles; and acting a thing out in miniature helps bring it about. It’s the oldest reason a spell uses a particular color, shape, or image.

Why do spells use specific colors?

Because a color is a likeness. We reach for red when we mean the heat of passion, green when we mean growth — the eye matching the shade to the wish. (The tidy modern “color chart” is a fairly recent convenience; the instinct to match the color to the meaning is ancient.)

Is a poppet the same as a “voodoo doll”?

The folk poppet is mostly a tool of care — a figure shaped to stand for someone so you can send them healing, comfort, or protection. The grinning doll-of-harm from the movies is the rare, dark exception, not the rule.

Is the walnut really medicine for the brain?

No — and that’s the honest part of the story. The old “doctrine of signatures,” which read a plant’s shape as a sign of its use, was a way of reading the world, not a proven pharmacy; the resemblances were usually noticed after a use was already known. We give it as a window into how the law was understood, not as medical advice.

Sources

The modern name comes from James Frazer, The Golden Bough (1890), who called the principle the Law of Similarity — “like produces like” — and named the magic built on it “Homoeopathic or Imitative Magic,” one of the two branches (with Contagion) of “Sympathetic Magic.” Isaac Bonewits keeps the same Law of Similarity in Real Magic (1971): “effects are liable to have one or more outward physical or inward mental appearances similar to one or more of said appearances of their causes.” There is no older single name for the principle; we use Frazer’s.
The walnut-for-the-brain is the famous example of the Doctrine of Signatures, set down by the English herbalist William Coles, Adam in Eden (1657); the idea was gathered earlier by Giambattista della Porta, Phytognomonica (1588), and named by Jakob Böhme, Signatura Rerum (1621). Modern scholarship treats it as a worldview, not a medicine: the resemblances were read onto plants after a use was already known (B. Bennett, Economic Botany, 2007).

Listen to the Podcast

Our podcast goes deeper — a plain-English conversation about why spells use color, shape, and image, and the one old idea underneath it. This is not a reading of the post.

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