One of the few books I've read all in one sitting is Metaphors We Live By, by George Lakoff. It was so fascinating learning how some of the most fundamental concepts we use to govern our lives are understood through metaphor. For example, one understanding of time is through our experience of space, so it seems natural to say, the hard times are behind us, or, I'm looking forward to the future. Or that Ideas Are Objects so you can give someone an idea, or toss a few ideas around. Or that Understanding Is Grasping, so you can get the idea, or get to grips with a concept.
Primary metaphor is a theory of how these metaphorical understandings may form when we're young as we associate abstract ideas with direct experience.
So it's natural that, when we're young, important things are often big and can exert control over us, leading to Important Is Big — Tomorrow's a big day, it's just a little thing, It was a huge deal.
Or experiencing warmth when we feel affection leading to Affection Is Warmth — he greeted me warmly, she gave me the cold shoulder.
Or how sometimes to know something is to see something, such as when we find out what's in a box by seeing what's in a box, leading to Knowing Is Seeing — I see what you mean, that's clear to me know, I'm feeling in the dark on that, let's shed some light on the matter.
Other examples of primary metaphor from Philosophy in the Flesh by Lakoff and Johnson include:
There are many more, plus the whole world of metaphor guiding how we construct our place in the world is fascinating.
The theory of primary metaphor is from Joseph Grady.