I am studying analog design, and my instructor told me that you could determine whether or an amplifier is stable without knowing the transfer function, or the circuit diagram, just by examining the Bode plot. Below is an example of an unstable system. He claimed that it is unstable because there is non zero gain at the 180 degree phase shift point. My understand is that this only unstable if there is feedback, which we don't necessarily know.
My question is this: How do we know if the system is stable from just the Bode plot?