"Spontaneous", "voluntary" and "involuntary"

A voluntary action is one that the actor, employing free will, chooses to take.

An involuntary action is one that takes place out of the control of the actor, or against the actor's will.

Spontaneous has several related definitions. A spontaneous action can be one that the actor takes without a significant amount of forethought ("a spontaneous purchase"). In that case, the spontaneous action is voluntary.

On the other hand, spontaneous can also refer to something that occurs with no apparent cause ("spontaneous combustion"). By its nature, this activity rarely involves an overt "actor" per se. When it involves a living organism ("spontaneous growth", "spontaneous motion", etc.), the action is probably involuntary.

Spontaneous has a few other definitions as well. Generally, you'll have to discern the difference from context. When you see the word "spontaneous" used to describe an action taken by a clearly definable actor, though, the action is probably voluntary (although it might not be).

Even though voluntary is the opposite of involuntary, they do have something in common. Consider the following meanings:

• Voluntary describes an action taken by an actor without being compelled to do so by someone else.
• Involuntary describes an action performed without the actor's will.
• Spontaneous is something that happens without intervention of something else.

All three cases involve something which does not interfere with the action.

• voluntary: EXTERNAL ENTITY does not interfere with ACTOR performing ACTION.
• involuntary: ACTOR does not interfere with ACTION

So both are spontaneous, but with a different non-interfering part.