How much we know

We have the impression that mankind knows a great deal.  This impression may come from the fact that for 12 to perhaps 24 years we are taught mankind's collected knowledge in our schools.  Year after year we hear of mankind's accomplishments.  But we typically only hear of the things mankind does understand.  As you might expect our teachers (wisely) have only a little to say of those matters we don't understand.  So our impression is biased and may be wrong.