Wealth is the value of everything you have less what you owe. Every item can be valued by seeing what people would pay for it, which is usually just its current market price. How do you determine income objectively? You could merely take wealth at two points in time but our society has declined to define it that way because much of the money coming in might go to some sort of expense which ought to be excluded, which is neither objective or simple, and because some changes (such as fluctuations in asset prices) are assumed to be transitory and therefore impractical to tax. The latter is actually the source of most of the perfectly legal tax evasion in this country: Hold stocks, generate value, pay nothing until you sell which needn't be ever.