Fictions contracting with Non-fictions

i know the whole basis of using the fact that u are not a person or other artificial entity is based on the fact that fictions (governments) are not supposed to be able to contract with nonfictions (men and women) but only other fictions. what is the maxim of law or case law for this important basis? ive known about this defense for years. the only thing i remember is sovereigns are not "persons". is that the only case law (the church of scientology case) are is that also a maxim of law that you have to be on the same legal standing to bring suit?