The next section, s. 175, though doubts have sometimes been expressed as to its application, at least shows that "assessment" is regarded as a process producing a legal effect. The provision is that the "validity" of any assessment shall not be affected by reason that any of the provisions of the Act have not been complied with. I shall not refer in detail to ss. 185 to 202 inclusive, which deal with reviews and appeals, but the language of those sections cannot, I think, be reconciled with any other view than that without a notice of assessment fixing a taxable income and a tax there is no assessment. Indeed it would be hard to find clearer indications of this then s. 170 itself provides. The topic of the section is the power of the Commissioner to amend assessments, a concept which pre-supposes that an assessment is something creating a legal obligation, so that any amendment of it must depend upon a positive grant of power to alter that obligation. Throughout the section an assessment is referred to as a specific, identifiable thing, which, unless amended (or of course affected on review or appeal), will stand as decisive of liability. To speak, as does sub-s. (1), of tax being paid in respect of the assessment, or to speak, as do sub-ss. (2) (b), (3), (4), (5) and (6), of tax having become due and payable under the assessment, would be impossible unless "assessment" meant the whole process which comes to a head in the service of a notice of assessment and thereby becomes, as a whole, an act in the law. If this be correct, it follows that until a notice of assessment has been served on the taxpayer the Commissioner and his officers neither need statutory authority to go back over any or all of the steps that have been taken in the office, and correct anything they consider to be erroneous, nor are disabled from doing so by anything in s. 170.