Fluid/Crystallised

Consider intelligence in two frames:  fluid or crystallised. 

Fluid intelligence is ‘idea-generating’, while crystallised intelligence is “making links and using knowledge” rather than creating it.

People with experience (i.e. age) use crystallised intelligence to improve the known.  For example, salespeople hone their pitch, or government workers get through red tape. 

Our brain is a library accumulating books, which feeds crystallised knowledge. 

Fluid intelligence figures out which books to accumulate, and the crystallising is reading, adopting, and adapting that knowledge.

The science shows that our brains change in mid-life.  We rely more on crystallised knowledge because we’ve built up our library.