Imagine yourself a child. Imagine yourself given one of these books: not merely a story of exploration and adventure, but a manual to such. Imagine yourself acquiring the keys to a mutable world in which you can explore caves, fight spiders, build castles, ride pigs, blow up mountains, construct aqueducts to carry water to your summer palace… anything. Imagine yourself a child, in possession of the secret knowledge.

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The secret of Minecraft — The Message — Medium

This article is the best explanation for Minecraft’s appeal I’ve seen.