Question: I was looking for a synonym of *spontaneous*, and *voluntary* naturally came to my mind. In an attempt to understand the difference between them, I tried to google *spontaneous vs voluntary*. To my surprise, nothing really interesting popped up from search results.

Then I decided to look them up respectively. As expected, *voluntary* is listed as a synonym of *spontaneous* according to many online resources, and vice versa. However, to my great shock, *involuntary* is also listed as a synonym of *spontaneous* by major online dictionaries.

Although the fact does not necessarily imply *voluntary* and *involuntary* are synonyms, I continued to look further into *voluntary vs involuntary* due to confusion.

I found that a *voluntary* action is something that is done voluntarily or with meaning to do so, while an *involuntary* action on the other hand is done
*automatically*.

Now I'm even more confused. The word *automatically* just reminds me of *spontaneously*, convincing me to believe *involuntary* is indeed a synonym of *spontaneous* while *voluntary* seems less so.

Could someone please justify or explain the contradictions mentioned above? I'm totally lost.

Answer: 
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.