Declaration of will, which normally depends on the will of the person which submits it, can also be the subject of the legal duty. Substantive source of compulsory realization of this duty is article 64 k.c., which states: valid court’s sentence that ascertains someone’s duty of placing specified declaration of will, replaces that declaration. Normative content of this rule manifests in establishment of legal fiction, which states that Court sentence, based on the norm from article 64 k.c. is tantamount to statement that certain person’s declaration of will (with content from court opinion) was stated in fact.

Most common example of quoted before rule’s application in practice is claim for definite person’s obligation to placing the declaration of will that demise the ownership or transfer the perpetual usufruct of immovable to another person. That claim results from preliminary contract of sale, which is coming into effect on the recourse to law.

Analysis of the article’s 64 k.c. content lead to a conclusion, that court opinion can substitute the declaration of will in the meaning that, it causes the same results, which will appear by declaration of obliged. (E. Gniewek, Kodeks cywilny, Komentarz, wyd. 2, Warszawa 2006, s. 152).

When the obliged’s declaration of will is going to be the only one element of the contract, coming this agreement to conclusion depends on second side’s correct form of declaration. However, mentioned before rule has one meaningful breach which spears in judicial decisions: taking into consideration the demanding to conclude promised agreement in preliminary contract. As Supreme Court states in sentence at 19 September 2002 in case with signature II CKN 930/00 ”If that declaration of will is a part of agreement committed by parties, to contract that agreement there have to be placed certain declaration of will by second party in correct form. There are few exceptions to that rule which are connected to promised contract (art. 390 § 2 k.c.) and to situation, when court will allow the claim, that contain the duty of contract’s conclusion which is completely consistent with party demand. In this cases court opinion states concluding the agreement and replaces this agreement”.

Substitute function of a sentence based on article 64 k.c., puts itself into practice ex lege at the moment of its implementation, so there are no necessaries of any additional declarations of the will in a form of notarial act (as Supreme Court states in sentence at 5 September 2002, II CKN 1035/00, LEX nr 75263). So there is no need of compulsory execution of that statement, which is in accordance with disposition form article 1047 §1 k.p.c, which correlate with article 64 k.c. (A. Jakubecki (red.), Kodeks postępowania cywilnego. Praktyczny komentarz, Zakamycze, 2005, wyd. II.). It is generally considered, that in case of the lack of mentioned rule, the debtor, due to reaching results of the sentence, will be forced to execute the performance of the act, which can not be performed by another person in the name of debtor and which realization depends only on him. (K. Piasecki (red), Kodeks postępowania cywilnego, Komentarz, Tom II, Wyd. C.H. Beck, Warszawa 2006, s. 1202)

Very important exception to the moment of beginning the result of court statement, which states the duty of placing the declaration of will is connected to the article 1047 §2 k.c. and is related to reciprocal obligations. In situation when placing the declaration of will depends on creditor’s reciprocal obligation, the result mentioned in previous paragraph begins at the moment of legal conferment the court’s statement with executory formula. Ipso facto, taking into consideration the content of the article 786 §1 k.p.c., the court can give the executory formula to the statement only after creditor’s proof (which can be official document or private document with legalized authentication) that he accomplished the reciprocal obligation (especially paid the sale price). The result, which will be the fiction of debtor’s declaration of will with content specified in sentence, begins at the moment of executory formula decision validation. Simultaneously, after the validation of this statement, creditor has the possibility of revealing his law in land and mortgage register, which is manifested by an entry in that register (article 1048 k.c.). It is worth to remember, that in case of transferring the perpetual usufruct, material result will begins at the moment of entry in land and mortgage register, which has constitutive status (vide: statement of Supreme Court, 15 April 2004, IV CK 272/03, LEX nr 176082).