The binding legal principles established are: (1) Section 22(1)(a) of the Income Tax Act requires trading stock to be valued at cost price as the baseline, not at net realisable value or market value; (2) The Commissioner may only make a just and reasonable allowance where stock has diminished in value, meaning where it is no longer worth what was paid for it; (3) Taxable income must be determined on a year-by-year basis based on events that have actually occurred during that tax year (backward looking), and valuation methods that are forward looking (such as net realisable value) are inconsistent with this principle; (4) Expenses may only be deducted in the tax year in which they are incurred in the production of income for that year, and valuation methods that effectively allow future expenses to be deducted in an earlier year are impermissible; (5) Accounting standards such as GAAP and IFRS do not determine the proper tax treatment of trading stock where the Income Tax Act prescribes a different methodology.