No Silver Bullet: Essence and Accident in Software Engineering Fred Brooks, 1987.
Brooks considers the difficulties encountered in building software and divides them into two kinds: essential difficulties, those inherent in the nature of software; and accidental difficulties, where we make things harder for ourselves than we need to, but that are not inherent to the process. Accidental difficulties we can chip away at, but essential difficulties will always remain.
Source: https://blog.acolyer.org/2016/09/06/no-silver-bullet-essence-and-accident-in-software-engineering/