Berry, J.
With reference to assessments for grading streets, the charter of St. Paul (Sp. Laws 1874, c. 1, § 25, p. 62, as amended by Sp. Laws 1875, c. 1,) provides that, before proceeding to make an assessment, the board of public works shall give notice of the time and place of their meeting for the purpose of making the same, “ in which notice they shall specify what such assessment is to be for, and the amount to be assessed, ” and that all per*445sons interested in any such assessment shall have the right to be present, and to be heard either in person or by counsel, and the board may, in their discretion, receive any legal evidence. Section 26 provides that when the board have completed the assessment, they shall give notice of a meeting to hear objections, and for the confirmation of the same, at which meeting all parties interested shall have the right to appear, and show cause why the assessment should not be confirmed;- and the board are authorized, in their discretion, to revise and correct the assessment, and to confirm or set it aside. The city treasurer, to whom, the warrant for the collection of the assessments is delivered, having given notice of his intended applications for judgment for delinquent assessments, and having made report to the district court, the court proceeds to the hearing of the applications, and the owner of any property reported delinquent, or any person interested therein, may appear and file objections in writing to the recovery of judgment against such property. “No objection,” says section 39, as amended, “shall be interposed or sustained in relation to any of the proceedings prior to the confirmation of the assessment, except that the common council had no authority to order the said improvement, or that the board of public works had no authority to have the said work performed; and no objections as to any other of the proceedings shall be sustained on any mere formal irregularity or defect; and the city treasurer may amend, by leave of the court in its discretion, in any matter in furtherance of justice. The court shall hear and determine all objections in a summary manner, without pleadings, and shall dispose of the same with as little delay as possible, consistently with the demands of public justice; but should justice-require that for any cause the suit as to .one or more owners-should be delayed, judgment shall then be rendered as to the-other property and lands, and process shall issue for the sale thereof the same as in all other cases.” Section.54, as-amended, provides that “ no error or omission which may have *446heretofore been, or may hereafter be made, in' the ordér or in' the proceedings of the common council or board of public works, or of any of the officers of said city, in referring, reporting upon, ordering, or otherwise acting, concerning any local improvement provided for in this chapter, or in making any assessment therefor, or in levying or collecting such assessment, not affecting the substantial justice of the assessment itself, shall vitiate or in any way affect such assessment.”
The charter provisions relating to such local improvements as the grading of streets show that, before any proceedings are had for making or confirming assessments, the questions of the propriety and necessity of the improvement, and whether there is property the benefits to which will' equal the expense of making it, have already been passed upon and determined by the common council and board of public works. This determination includes, of course, a determination that an assessment is necessary and proper. The improvement having thereupon been ordered to be made, and the contract let, as by the charter provided, the questions next to be considered are, what parcels of property are benefited,'and what is the benefit to each parcel proportionate to the expenses of the improvements? With reference to these questions two opportunities are afforded all interested parties to be heard before the board of public works. The theory of the charter appears to be-that at the first hearing the board may properly hear suggestions, arguments, and, in their discretion, evidence, as to what area and what parcels of land are specially benefited by the improvement, and which should therefore be assessed, and as to the proportionate amounts to be assessed upon the different parcels benefited. After this hearing the assessment is completed, •as the charter says, — that is to say, it is made out. Then follows a second hearing of objections to the assessment, and •for its confirmation. At this hearing all parties interested ¡are entitled to be heard upon the question of the propriety *447and justice of the assessment as made out by the board. This is the first occason on which such parties are enabled to criticise the assessment, because it is the first occasion on-which they have been notified and permitted to meet it as it is actually made out. While the charter does not provide for the adduction of any evidence before the board at the second hearing, as it does at the first, still the whole question of the propriety, fairness, and justice of the assessment, as it is made out, is up and open for objection, though the question as to whether there is occasion for cm assessment is not open — that, as we have before seen, having been previously settled. Now, when the report of the city treasurer comes into the district court, any person interested may appear and file objections to the recovery of judgment. Objections relating to proceedings prior to the confirmation of the assessment can go only to the authority of the common council to order the improvement, or to the authority of the board of public works to have the work performed; but as to the confirmation of the assessment, the right to interpose objections is unlimited, except that no merely formal irregularity or defect, or any error or omission which does not affect the substantial justice of the assessment, vitiates or in any way affects it. We can conceive of no proper objection going to show that the assessment ought not to have been confirmed, as respects the objector’s property, which cannot properly be made upon the hearing of the application for judgment in the district court.
We use the phrase “proper objection” advisedly; for we are not to be understood as holding that any objection which a person interested sees fit to urge as a reason- why the assessment should not have been confirmed, will be considered by the district court. It would be unwise in the extreme for us to undertake in advance to enumerate in detail all the obj ections which would or would not be proper obj ection s. But we may go so far as to say that an obj ection that the assessment *448•was made with fraud in fact on the part of the board, as is-claimed by the relators to have been the case in the proceedings which are sought to be brought before us at this time, would be a proper objection. But an objection which went no further than to claim that the judgment of the board of public works upon the matter of assessment differed from the judgment of some other person or persons, would certainly be an improper objection.
The questions, what property is benefited by the improvement ? and how much ? are questions of opinion upon the facts as they appear. They are therefore questions, the decision of which cannot be regulated by any quasi mathematical rules of law. They must be left to the judgment of men. The charter has committed them to the board of public works, and their judgment is final and conclusive, and cannot be reviewed by the district court or any other tribunal, unless shown to be fraudulent in fact, or unless it is made up upon a demonstrable mistake of fact. We say a demonstrable mistake of fact, meaning by this expression a mistake of fact as to the existence of which there is no room for doubt. An instance of such mistake would be this: Suppose that on each side of a street, graded under the charter provisions which we are considering, there were five blocks' in every respect alike, except in location. The assessment of four on each side, leaving the middle block on each side unassessed, if -it were not a case of fraud, would be an instance of demonstrable mistake of fact. The failure to-assess the middle blocks could not be accounted for on any other basis. We will not pursue this subject further.
In the case at bar, the complaint of the relators is confined to the assessment. As before stated, we are of opinion that any objection which can properly be made to the assessment can be made upon the hearing of the application for judgment in the district court/ If this is so, the exclusive remedy of the relators is upon such hearing in the district court, with *449a right to review the action of the district court in this court. It follows that, for the purpose of attacking the assessment, a certiorari will not lie to the common council or board of' public works.
The writ herein issued is accordingly quashed.