one's own volition; one's own will
A formal phrase emphasizing that an action is taken by one's own free will, not under compulsion. Often used in legal or official contexts.
He entered into the contract of his own volition.
It is important to act based on one's own will.
自由意志 is a philosophical term for 'free will', while 自己の意思 is a more concrete phrase meaning 'one's own volition' in specific decisions.
自発的 is an adjective meaning 'voluntary' or 'spontaneous', whereas 自己の意思 is a noun phrase focusing on the source of the decision being oneself.
Compound of 自己 (self) + の (possessive particle) + 意思 (will, intention). The phrase is a straightforward combination of these elements.