My personal recommendation just based on personal experience would be to study philosophy. It doesn't much matter which philosophies, but a variety that differ fairly substantially would probably be best. When you read a very different viewpoint you can occasionally almost feel your brain shift sideways when you 'get' how the other viewpoint is seeing things. That leads to questions about the exact points where your viewpoint and their actually diverge, and considering a wider scenario where both exist or develop is inherently a move in the abstract direction.
