If you want to be a successful leader, and especially if you want to run a successful company, there's one really, really tough question you have to ask yourself often. It's about prioritization, perhaps the most difficult thing for all of us to do, both in business and in life.

It was 2007. Seven years after the dot-com boom and then bust. A year or so before the financial crisis. Yahoo CEO Terry  Semel had just resigned, having failed miserably in his attempts to purchase first Google, then Facebook. The  Yahoo board appointed co-founder Jerry Yang as CEO.

A who's who of Silicon Valley figures offered to help Yang in his new role. One of them was Steve Jobs, who arrived at Yahoo to deliver a speech to hundreds of its upper managers. The subject of his speech was prioritization. 

Jobs described how he'd returned to Apple to lead the company 12 years after being forced out. He returned to find too many products and product lines. So he set his managers to work on prioritization, on focusing on what was more valuable and important for Apple and letting the rest go.

Jeff Weiner was sitting in the audience. At the time, he was a Yahoo executive; within two years he would move to LinkedIn, where he is now CEO. All these years later, Weiner still hasn't forgotten that speech. "Prioritization sounds like such a simple thing," he told The New York Times in 2012. "But true prioritization starts with a very difficult question to answer... If you could only do one thing, what would it be?"

You can't cheat when answering this question, Weiner says--for instance, you can't answer by attaching one thing to another. "I was struck by the clarity and courage of his conviction," Weiner recalls. "He felt it so deeply, and there wasn't a person in the audience that day who did not take that with them as a lasting memory."

How many times have you said "no" today?

Jobs' longtime friend, Jony Ive, chief design officer at Apple, recalls getting a similar lesson from Jobs. "Jony, you have to understand there are measures of focus, and one of them is how often you say 'no,'" Jobs said to him, according to the website Ladders. Jobs himself was so famously committed to focus that he dressed in the same black designer turtleneck, sneakers, and jeans every day, eliminating the need to think about what to wear.

Ive also said Jobs would often ask him how many things he had said "no" that day--a question Ive found patronizing, although he agreed with the principle. "The art of focus is, even if it is something you care passionately about, focus means ignoring it, putting it to the side," he said. "And often, it's at real cost. And [Jobs] was remarkable at that. "It takes so much effort and is exhausting to sustain, but all of the good things we've done have required that sort of focus."

So ask yourself: If you could do only one thing, what would it be? Once you have a clear answer to that question, ask yourself this one: Are you devoting most of your time and attention to that one thing right now? And if you aren't, why aren't you?