Here are six defining traits that keep surfacing over and over again in leadership bestsellers.

What are the defining attributes of great leaders? That’s the age-old question thought leaders and scholars galore have been attempting to answer in mountains of books and literature.

While great leadership, to an extent, can be personal and subjective to the follower, there are universal principles you can’t argue with (but you can try). Speaking of those thought leaders and scholars, here are six traits that keep surfacing over and over again in the leadership literature and best-sellers.

1. They challenge their own assumptions.

Great leaders may be smart and know a lot, but they are humble enough to recognize there are smarter people in the room that they can learn from. They don’t restrict themselves from opinions and input outside of their own. They surround themselves with diverse perspectives to help them answer questions like, “How do I know my decision is the right one?” or “Is there a better course of action here?”

2. They are radically transparent and model it for others.

Transparency promotes an open culture of respect, openness, and dignity void of the usual toxic corporate metaphors like backstabbing, gossip, and throwing people under the bus. The business case for it has and always will be about the team — about strong relationships, collaboration and, lest we forget, getting results. But transparent leaders go beyond self-transparent behaviors: They allow others to voice their opinion and encourage emotional honesty and uncomfortable conversations in board rooms and conference rooms.

3. They are learning machines.

Great leaders recognize that we are in an age of unprecedented technological advancement. They develop their own competency by continuously learning and gathering expertise across multiple fields, not just their own. They also champion a “learning spirit” within the organization, sending a clear message to knowledge workers that “growing our people is one of our highest priorities.”

4. They have mentors and pick them carefully.

Great leaders surround themselves with sages they can approach for wisdom and honest feedback. They also choose their mentors carefully because receiving advice from the wrong people could potentially be career-limiting and a bad move. They find tried-and-true mentors with a high degree of integrity they, and others, admire and would like to emulate.

5. They build strong relationships.

Leadership practitioners like myself preach this ad nauseam, yet it often falls on deaf ears. For those that do value authentic relationships, they’ll see tremendous differences in how employees and customers alike respond. Margaret J. Wheatley, renowned management consultant and author of Leadership and the New Science, which has been lauded as “one of the top ten business books of all time,” gave timeless advice to point new leaders in the right direction: “We will need to become savvy about how to build relationships, how to nurture growing, evolving things. All of us will need better skills in listening, communicating, and facilitating groups, because these are the talents that build strong relationships.”

6. They serve others.

Leadership is not dictating, commanding, or imposing. It is being of service to others (yes, to your customers but especially to your employees). It is empowering others to achieve their goals, bringing out the best in people, putting their needs ahead of your own, and helping people develop and reach their highest potential. We call this servant leadership–one of the highest platforms to launch your leadership success.

This article was originally published at Inc.com.