A motu proprio
"on his own impulse") is a document issued by the Pope
on his own initiative and personally signed by him.
It may be addressed to the whole Church, to part of it, or to some individuals.
The first motu proprio was issued by Pope Innocent VIII in 1484. It continues to be a common form of rescript, especially when establishing institutions, making minor changes to law or procedure, and when granting favours to persons or institutions.
An important effect of the issuance of a document in this way is that a rescript containing the clause "motu proprio" is valid and produces its effect even in cases where fraud would ordinarily have vitiated the document, since the Pope does not rely on the reasons alleged when he grants a favour. Withholding of the truth in what, according to canonical law, style and practice, must for validity be expressed, normally renders a rescript invalid, but not if the rescript is issued "motu proprio". Consequently, canonists traditionally called the clause the "mother of repose".
However, a motu proprio has no effect in so far as it harms the acquired right of another or is contrary to a law or approved custom, unless it expressly states that it is derogating from these matters.
A motu proprio rescript begins by giving the reasons for issuing it, and then indicates the law or regulation made or the favour granted. It is less formal than a constitution and carries no papal seal. Its content may be instructional (e.g., on the use of plainchant), administrative (e.g., concerning a church law or the establishment of a commission), or merely to confer a special favour.
Other uses of the phrase "motu proprio"
More generically, the Latin phrase is used to indicate "of his own accord" and is thus similar to "sua sponte". It is used very rarely in legal opinions in the United States: the better known term "sua sponte" is preferred.