Simon Sinek defines "the why" as a purpose, cause, or belief that motivates us, separate from the generic question "why." He explains he chose the term because vision, mission, and purpose were used interchangeably but all pointed to the same underlying idea.
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So a lot of people write in and ask the difference between the question why and the why as we talk about it. Like, start with why. The why is a purpose, cause, or belief. The underlying reason why we are motivated to do something. The reason a company exists, the reason why we're passionate for something. The question why aims to get at the same thing in a generic sense, like why do you do something? What's the underlying reason for that? That's why I called the why the why. Another reason why I called the why the why is because when I would ask people what comes first, vision or mission? People would debate it. There's no standardized definition of vision or mission, so people who believe mission came first. I asked them, so what's the definition of mission? They said, it's why we get out of bed in the morning. People who believed vision comes first would say, well, it's why our company exists and whether it's brand or purpose, all these things. Everybody gave me the same definition why, so that's why I called it the why.