Mo. Rev. Stat. § 521.060: Form of affidavit, by whom made.

The affidavit shall be made by the plaintiff, or some person for him, and shall state that the plaintiff has a just demand against the defendant, and that the amount which the affiant believes the plaintiff ought to recover, after allowing all just credits and setoffs, is ______ dollars, and that he has good reason to believe, and does believe, in the existence of one or more of the causes which according to the provisions of section 521.010 would entitle the plaintiff to sue by attachment.