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 asset comes down to Rs. 90,000, it will ordinarily continue to be show at Rs. 1,00,000 and not at Rs. 90,000 due to _______________.