Imagine you have an infinite sheet of neutral charge in the (x,y) plane, spanning all of (x,y).
Place a positive charge at the position z = a.
This positive charge induces a negative charge on the sheet, everywhere in the (x,y) plane.
This gives rise to two scenarios, neither of which makes sense.
>1. The sheet originally had an equal amount of positive and negative charges, hence being neutral
If the sheet is now negatively charged everywhere, where the fuck did the positive charges go?
>2. The sheet was originally neutral because it had no positive or negative charges anywhere
So where the fuck did the negative charges come from?