The provisions of Sixth Council Directive must be interpreted as precluding national legislation, such as that at issue in the main proceedings, by which a taxable person is not allowed to deduct the value added tax due or paid in respect of goods that were delivered to him on the grounds that the invoice was issued by a trader which is to be regarded, in the light of the criteria provided by that legislation, as a non-existent trader, and that it is impossible to determine the identity of the actual supplier of the goods, except where it is established, on the basis of objective factors and without the taxable person being required to carry out checks which are not his responsibility, that that taxable person knew, or should have known, that that transaction was connected with value-added-tax fraud, this being a matter for the referring court to determine.