In 1956, George Miller wrote a paper that suggested that the number of objects that a human mind can hold is 7 plus or minus 2.  Here’s a wikipedia article that explains it.   You mind can hold 7 or so pieces of information at one time.

That was over 50 years ago.  Today, that number is significantly less, and the idea now is that the mind can only hold 2 plus or minus 2 pieces or ideas at once.  Whether it’s education and learning or formation, the mind can only handle so much.  The attentional ability of the mind can be so overloaded that it prevents the mind from deeper thinking.  You might be able to hold more than 4 pieces of information on your mind, but you are only able to do so shallowly.