But, even were it true that the commissioner must, upon the hearing of the appeal, affirmatively prove by evidence that he formed a judgment of the amount of the income upon which the appellant ought to be taxed, it could not be part of his case to establish the facts upon which he acted in forming the judgment or the grounds on which he proceeded, the materials before him, or the reasoning actuating him. The need supposed of showing that he formed such a judgment could be no ground for requiring particulars of the sources of the taxable income ascribed by the assessment to the appellant. The assumption made, however, has no foundation. The formation of the judgment as to what is the amount of the income that ought to be taxed is no condition precedent to the power to assess. It is part of the very process of assessment itself. Section 166 and s. 167 do not prescribe distinct duties or functions. They combine to show what the commissioner may or must do in performing his single duty of arriving at an assessment. Section 166 on its own terms covers cases where the commissioner depends exclusively on sources other than a return. It says that he is to make his assessment from (1) the returns, (2) from any other information, or (3) from any one or more of these sources. Clearly enough under s. 166 the commissioner can make an assessment which does not adhere to the income returned and yet to do so must involve some want of satisfaction with the return. Section 167 is epexegetical to s. 166. It is not an independent power. What it does is to mention with particularity three situations which might arise in carrying out the duty imposed by s. 166, and to direct how in those situations the commissioner shall proceed for the purpose of s. 166. Just as under s. 166 considered alone the commissioner ascertains the amount of the taxable income and thus assesses it so does he under s. 167, used in aid of s. 166, ascertain the amount upon which, in his judgment, income tax ought to be levied and thus assesses it. By definition "assessment" means the ascertainment of the amount of the taxable income, and of the tax payable thereon. This is the view of ss. 166 and 167 adopted by Williams J. in McEvoy v. Federal Commissioner of Taxation [1] . The fact is that unless the taxpayer discharges the burden laid upon him by s. 190 (b) of proving that this ascertainment or judgment is excessive, he cannot succeed and it can be no part of the duty of the commissioner to establish affirmatively what judgment he formed, much less the grounds of it, and even less still the truth of the facts affording the grounds. Yet that is what is involved when the demand for particulars of the sources alleged of the appellant's income is justified by reference to s. 167. It is an error to treat the formation by the commissioner of a judgment as to the amount of the taxable income as if it were not the ascertainment of the taxable income which constitutes assessment or a necessary part of that process and as if it were but the fulfilment of a condition precedent to the power or authority to assess. If, however, it were a condition precedent the question would at once arise whether the fulfilment of the condition was not part of "the due making of the assessment" of which s. 177 (1) makes the production of a notice of assessment conclusive evidence. But of this it is unnecessary to speak specifically. It is unnecessary to do so not only because the reasons already given are enough to dispose of the request for particulars of the sources of the appellant's additional taxable income, but also because a similar question must now be dealt with in relation to the additional particulars which the appellant seeks in reference to the year of income ended 30th June 1947, that is to say, particulars as to the person or officer who formed or made a judgment under s. 167. The demand for these particulars arises from the view for which the appellant contends that under s. 167 (b) it is essential that the want of satisfaction with the return should exist in the commissioner or second commissioner and not a deputy commissioner. If, however, it may exist in a deputy commissioner, at all events, so he contends, it cannot be enough that it exists in some lesser officer to whose opinion the deputy commissioner gives effect by authenticating the assessment. Presumably in the same way the judgment as to the amount of income to be taxed must be formed either by the same higher officer or one of like status. Accordingly the appellant by his summons seeks particulars of the person or officer who formed or made a judgment under s. 167. This, doubtless, was meant to cover the failure to be satisfied under par. (b), as well as the formation of a judgment as to the amount of income on which tax ought to be levied.