How can a negative multiplied by a negative give positive? On first look this can seem weird. But I can explain what I am looking for.
We all know from elementary maths that $(-\times-)=+$. 
Now, lets say there are 3 cows and I say they will become doubled after one year so $3*2=6$.
And lets say I have $-3$ cows(which is not possible, because I can show $3$ cows but not $-3$) and if I multiply it by $-2$, I get $-3\times -2=6$. How is it possible?, nothing multiplied by nothing equals something?
Some said the reason belongs to philosophy, if yes, what's the idea behind it?

Multiplication by a positive number is rather intuitive, so $(+ \times +)$ and $(+\times -)$ are relatively easy (for the latter, think you multiply a debt, for example). So the hard one is $(- \times -)$. I may be uneasy to understand this directly, but if you want to keep laws of arithmetic as they are with positive numbers, then
$$a \times (b + c) = a \times b + a \times c$$
With $c=-b$, it yields
$$0 = a \times 0 = a \times b + a \times (-b)$$
$$a \times b = - (a \times (-b))$$
Now, if both $a$ and $b$ are negative, you must have on the left $(- \times -)$, and on the right, the negation of $(- \times +)$, thus a positive number.
