The basic question which motivated me to write this post is the following,

> 
> What is(are) the difference(s) between an axiom scheme and an axiom?
> 
> 
> 

In Margaris's book *First Order Mathematical Logic* we have the following, [![enter image description here](https://i.stack.imgur.com/vmsQz.png)](https://i.stack.imgur.com/vmsQz.png)

However, the difference between an axiom and an axiom scheme is not clearly stated there in the sense that in the book it is not clearly specified *exactly* what is(are) the property (or properties) that help us to distinguish between an axiom and an axiom scheme.

**Question**
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> 
> What is an axiom scheme? Is an axiom scheme different from one of its instances? If so, then in what sense **exactly**? 
> 
> 
> 

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**Disclaimer**

* I had some discussion with [user21820](https://math.stackexchange.com/users/21820/user21820) regarding this question and one can see our discussion [here](https://chat.stackexchange.com/rooms/44058/logic).
* For a different and more philosophical version of the question see [here](https://philosophy.stackexchange.com/questions/45501/differences-between-an-axiom-scheme-and-an-axiom).