An economic definition of income would include all capital gains in taxable income as they accrue each year. To tax them in this way would be difficult for three reasons: first, the value of many kinds of property cannot be estimated with sufficient accuracy to provide a basis for taxation: second, most people would regard as unfair the requirement to pay tax on income that had not actually been realised: and third, taxation of accruals might force liquidation of assets to pay the tax. Thus capital gains are included in taxable income only when they are realised.