An OpAmp without any feedback just takes the potential difference between his 2 inputs and amplifies it by a certain factor. I know that in an OpAmp Circuit with negative feedback, we can make the assumption that the potential difference between his 2 inputs is ideally zero. I know this is just an ideal assumption, in reality there will be a small potential difference remaining. I also know that when we have an inverting amplifier-circuit, we can basically "adjust" the amplification to a value we want by the ratio of the 2 resistors.
However I'm wondering: Once we have that circuit, what gets amplified is the Input Voltage and NOT the differential-voltage between his 2 inputs anymore. Why is that? The OpAmp doesn't "know" about the feedback surrounding him, so why does he not amplify that really small remaining potential difference between his inputs?