portfolio theory assumption


I was reviewing some level 2 portfolio theory readings and it says that one basic portfolio theory assumption is that "as an investor you want to maximize your returns for a given level of risk. It goes on to say to adequately deal with such an assumption, certain ground rules must be laid.

First your portfolio should include all your assets and liabillities including things like your house, car , stamp collection, furniture , coins, art and antiques etc.”

I did not understand the idea and meanning of this assumption. can someone explain what the point is here.

all assets without any exclusion. If only few assets are included then there will a bias merely because of a reduced number of choices.

I believe this has to do with " all assets are marketable" assumption. Later you can relax this assumption to include the effect of nonmarketable assets (roughly what you quoted above) on the equilibrium CAPM for a particular investor.