What do the greatest thinkers think about the most?

A2A on Quora:  ​What do the greatest thinkers think about the most?

I think this sums it up very nicely. Great thinkers think about ideas. What kind of ideas?
A good place to being is with the kinds of questions Socrates liked to ask:

What is virtue? What is justice? What is the good?

Each of these questions provides an opportunity to think about a big idea.

Most everything around us is either the product of a big idea or is deeply connected to one or more great ideas:

Think about how often you encounter something that relates to such ideas as:
time, equality, beauty, friendship, love, courage, freedom, democracy.

These ideas are fundamental to our lives and they are the kinds of ideas that the greatest thinkers think about.
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But, they are not the sole property of great thinkers. Anyone can participate in the discussion of great ideas. Indeed, everyone should participate in the activity of thinking about great ideas. It is an important part of what makes us all human beings.