science

Speak of the devil

Speak of the devil is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Speak of the devil rather than just read about it. In short: "Speak of the devil" is the short form of the English-language idiom "Speak of the devil and he doth appear" (or its alternative form "speak of the devil and he shall appear"). The form "talk of the devil" is also in use in the United Kingdom.

Key takeaways

Speak of the devil belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Speak of the devil to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Speak of the devil from memory before moving on to harder problems.

Reference excerpt

"Speak of the devil" is the short form of the English-language idiom "Speak of the devil and he doth appear" (or its alternative form "speak of the devil and he shall appear"). The form "talk of the devil" is also in use in the United Kingdom. It is used when an object of discussion unexpectedly becomes present during the conversation. It can also be used about a topic that quickly becomes relevant, such as the onset of rain or a car breaking down. A cognate of this phrase appears in the 15th century Chinese novel Romance of the Three Kingdoms as 说曹操，曹操到 or "Speak of Cao Cao, Cao Cao arrives". In many cultures, a different dangerous person or creature is referenced in the phrase.

Serbian: "Speak of the wolf and he is at your door." French: "Speak of the wolf and you see its tail." Finnish: "Pirusta kun puhuu, niin sarvet näkyy." Czech: "We talk about the wolf, and the wolf is at the door." Tunisian: "Mention the cat, and it comes to you jumping." Tunisian: "How long is your life!" (when it's a person being talked about and they show up) Norwegian: "Speak of the sun, it will shine." Swedish: "When you speak of the trolls, they're in the hallway." Hungarian: "Emlegetett szamár," meaning approximately "Here is the mentioned donkey." Spain: "Talking about the King of Rome and through the door he comes." Korean: "A tiger will come if it is talked about."

References

Worked examples

Example 1 — a first encounter with Speak of the devil

Start with the simplest possible case. Write down what Speak of the devil claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Speak of the devil before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Speak of the devil ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Speak of the devil

In research: Speak of the devil appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Speak of the devil in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Speak of the devil is common in secondary-school and first-year university syllabi. It links to neighbouring topics Demonology, Destiny, English-language idioms, so understanding it makes those chapters shorter.
In everyday life: Look for Speak of the devil outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Speak of the devil in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Speak of the devil means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Speak of the devil out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Speak of the devil in simple terms?

Why does Speak of the devil matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Speak of the devil?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Speak of the devil.