Interpret the arrows in a circuit diagram of an OP-amp
The arrow simply define the voltage direction notation. It's really super easy:

--------------&gt;

|            |
attach     attach
voltmeter  voltmeter
"-" here   "+" here


As you can see, that's necessary – it's not inherently clear which way around you'd measure voltage, and if you have an equation like \$y=-Ae\$, it'd be ambiguous.

Having that arrow is also a necessity, because what you do, introducing a ground potential at the positive input of the opamp, can't always be done – absolute voltages don't exist – everything is potential relative to an arbitrarily chosen point. And the arrow does nothing but define "the voltage I measure here is between the potential at the start of my arrow, and its tip – so subtract the start potential from the tip potential to get the voltage".