If a machinery is purchased for Rs.1,00,000 the asset would be recorded in the books at Rs.1,00,000 even if its market value at that time happens to be Rs.1,40,000. In case a year after, the market value of this assets comes down to Rs.90,000, it will ordinarily continue to be shown at Rs.1,00,000 and not at Rs.90,000 due to _________________.