Essentially, it consists of 2 main concepts; of
Investment Returns + Speculative Returns
Dividend Yield + Earning Growth +/- Change in PE ratio
Investment Returns = Dividend Yield + Earning Growth
Speculative Returns is the change in emotion of people in the market on how much they are willing to pay.
John Bogle also admitted that no one can predict the future, however he advised investors to stay on course, don't move money here and there, and keep investment simple. These are very good advice.