As applied to courts, jurisdiction is authority conferred by law to hear and determine controversies concerning certain subjects. Jurisdiction of the subject matter is the power to hear and determine causes of the general class to which the proceeding in question belongs, and such jurisdiction must be found in and derived from the law. (People v. Ford, 289 Ill. 550; Sumner v. Village of Milford, 214 id. 388.) As applied to a particular controversy, jurisdiction is the power to hear and determine that controversy. (People v. Ford, supra;People v. Talmadge, 194 Ill. 67.) Jurisdiction of the subject matter does not depend upon the sufficiency of the pleadings or the rightfulness of the decision. (O'Brien v. People, 216 Ill. 354. ) The exercise of jurisdiction may be erroneous, but if the court has authority to decide the cause every party properly brought before it must make his defense, and the judgment can only be set aside by appeal or writ of error when such a remedy is available. If the court has jurisdiction its judgment can not be attacked collaterally. Sumner v. Village ofMilford, supra.
Section 37 of the Local Improvement act expressly confers upon the county court jurisdiction of any proceedings under the act. (Cahill's Stat. 1923, P. 442.) Jurisdiction of the particular proceeding was acquired in the manner prescribed, and the judgment confirming the special assessment *Page 486 is not questioned. In due course the completion and acceptance of the improvement and its cost were certified to the court in which the assessment was confirmed, and the issue of the validity and sufficiency of the certificate was directly and expressly raised by the objections filed. The county court clearly had the right to decide whether the certificate was valid and sufficient, and after a hearing it determined that issue adversely to appellees' present contention. Its order, by section 84 of the Local Improvement act, (Cahill's Stat. 1923, p. 458,) is made "conclusive upon all the parties and no appeal therefrom, or writ of error thereto, shall be allowed to review or reverse the same." By the filing of the certificate, the fixing of the time and place for the hearing thereon, and the giving of notice of such hearing, as required by the act, the court acquired jurisdiction to decide whether there was a valid and sufficient certificate, and having such jurisdiction its decision cannot be assailed collaterally. People v. Sperry,314 Ill. 205; Village of Elmwood Park v. Mills Sons, 311 id. 136;People v. Ford, supra; Harman v. People, 214 Ill. 454;Sumner v. Village of Milford, supra; Conlin v. People, 190 Ill. 400; Perisho v. People, 185 id. 334; Leitch v.People, 183 id. 569; Pipher v. People, 183 id. 436.
The various steps in a special assessment proceeding, from the filing of the initial petition in the county court to the final approval of the certificate of completion and acceptance of the work by the same court, are parts of the same proceeding. Error in the course of the proceeding should not operate to divest the court of its jurisdiction or to render its judgment void so that it may be successfully attacked in a collateral proceeding. In our opinion appellees' objections in the instant proceeding which attacked the validity of the county court's order of December 18, 1923, should have been overruled. *Page 487