There are topics with the same name but my question is not as abstract as in those.
My question is as follows: taken a generic definition like $x\;\mathbf{ is\; something}$ if $y$ it could be written like $x\;\mathbf{ is\; something}\Leftarrow y$ so everytime you have $y$ then you can deduce $x\;\mathbf{ is\; something}$. But if you start with $x\;\mathbf{ is\; something}$ as an axiom you could not deduce $y$ which should be deducible. Like if you have $x\;\mathbf{ is\; even}\Leftarrow x \equiv 0 \pmod{2}$ you SHOULD have $x \equiv 0 \pmod{2}$ starting by $x\;\mathbf{is\;even}$. That should be resolved if you say by saying $x\;\mathbf{is\;even}$ iff $x\equiv0\pmod{2}$ instead of if (so you would have $\Leftrightarrow$ instead of $\Leftarrow$) but I definitely feel that I'm missing something...
Can you help me? :D
Sorry for my bad English and if my question was stupid.
An integer is even if it is evenly divisible by twoand in many other places definitions are given like this. – Mega-X Aug 04 '18 at 20:17