All Changes are Extrinsic — Eruvin 81b
תלמוד בבלי מסכת עירובין דף פא/ב
שזכין לאדם שלא בפניו ואין חבין לאדם שלא בפניו
The Rambam, Introduction §18 to Part II of Moreh Nevuchim writes (Friedlander translation): "Everything that passes over from a state of potentiality to that of actuality, is caused to do so by some external agent: because if that agent existed in the thing itself, and no obstacle prevented the transition, the thing would never be in a state of potentiality, but always in that of actuality. If, on the other hand, while the thing itself contained that agent, some obstacle existed, and at a certain time that obstacle was removed, the same cause which removed the obstacle would undoubtedly be described as the cause of the transition from potentiality to actuality, [and not the force situated within the body]. Note this."
It is on the basis of this principle that the Rogatchover (Tzofnas Pa'anei'ach, Mahadura Tinyana 11b) explains the concept of zechiah — specifically, why it must be done with another party, through whom the original owner bestows the ownership of an item upon a third party: A person cannot effect the change on his own, as all change must be effected by an outside agent.
According to the Rogatchover, this principle explains many Halachic issues. For example, the reason why a person cannot testify against himself (see Kesubos 23b) is because testimony effects change, and all change must be extrinsic (Teshuvos Tzofnas Pa'anei'ach [Dvinsk] 1:3).
This is also the reason why a transfer of ownership in which the owner is compelled to give up ownership without compensation is invalid, while the same transaction with compensation is valid (see Bava Basra 48b): When compensation is provided there is an extrinsic agent of change; when it is not provided there is none (Tzofnas Pa'anei'ach, Mahadura Tinyana p. 60; seee more examples in Mefa'anei'ach Tzefunos 14:4).